This story appears in the August 13, 2018, issue of Sports Illustrated. For more great storytelling and in-depth analysis, subscribe to the magazine—and get up to 94% off the cover price. Click here for more.

In the following exclusive adaptation from ASTROBALL, SI senior writer Ben Reiter discusses how the usually keen Astros made one of their greatest mistakes—cutting current Red Sox slugger J.D. Martinez. Adapted from ASTROBALL: THE NEW WAY TO WIN IT ALL Copyright © 2018 by Ben Reiter. Published by Crown Archetype, an imprint of Penguin Random House LLC. You can order a copy now on Amazon or here.

Of the 250 pieces of luggage that Red Sox clubhouse attendants keep track of during every road trip, one is most important. “If we lose that bag, we’ll be in trouble,” says Alex Cora, the rookie manager of a club that as of Aug. 5 had a major-league-leading record of 79–34 and that was on pace for 113 wins, fourth most in baseball history.

The bag doesn’t appear to be anything special. It’s a dark green duffel with a team logo. Its contents appear to be a haphazard collection—elastic bands, weights, Frisbees, a small kickball—except to the man who knows what to do with them.

The duffel is known among the Sox as J.D.’s Bag of Toys, and it is the key to maintaining the majestic swing that has since 2014 allowed J.D. Martinez, Boston’s first-year slugger, to post an OPS of .951 that ranks third in the big leagues, and this season to threaten to win a Triple Crown as the centerpiece of the game’s highest-scoring offense. As of Aug. 5, the 30-year-old outfielder ranked first in the majors in home runs, with 33, and RBIs, with 93, and his .324 batting average was third in the American League.

Martinez’s iPhone X stores 2,739 videos, and at least 2,000 of them are recordings of the same thing: J.D. Martinez taking batting practice. “Hitting, hitting, hitting,” he says, as he scrolls through the files. “Hitting. Fortnite. Couple on a boat. Hitting, hitting, hitting.” The video he views most often is from his pregame cage session last Sept. 4, the night he hit four homers for the Diamondbacks against the Dodgers. He studies the clips constantly, to detect any hint that he might be deviating from the cuts he took that evening. Each toy in the duffel has its purpose—its way of getting him back there.

Take the kickball. During BP, Martinez holds it between his forward elbow—his left one—and his torso, as he faces the pitching machine. “If the ball drops too early,” he explains, “then my elbow’s pushing back in a way that it shouldn’t.”

It’s all part of his ongoing effort to fight off his old swing, his natural one, the one that threatens to return every day. It was the swing that nearly drove him out of baseball a half decade ago, and which caused his first team, the Astros, to make its biggest mistake as it tried to lift itself from the laughingstock not just of baseball, but of all of sports, to champions.

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During Martinez’s first few bus trips from his team’s hotel in Venezuela and its home ballpark early in the winter following the 2013 season, he didn’t understand why the driver always took a different route and always drove at terrifying speed, barely braking at red lights. Then a teammate on the Caracas Lions explained it to him. It was for the same reason that the men riding alongside the bus on motorcycles were strapped with semiautomatic rifles—to fend off hijackers.

Martinez was 26, and he had already spent parts of three seasons in the Astros’ outfield. He was accustomed to life in the big leagues, with its cushy transportation and reliable hot water. But he wasn’t playing in the Venezuelan league for its luxuries. He was trying to save his career.

Martinez had always believed he was doing everything right. Though he had been a 20th-round pick out of Fort Lauderdale’s Nova Southeastern University in 2009, he had blasted his way to the majors in just two years. In 2012, Jeff Luhnow’s first season as Houston’s GM, Martinez led the 107-loss club in RBIs, even if that only required 55 of them. But over three years and 252 games, he had hit .251 with 24 homers. Each season, his production had declined.

One day in July 2013, Martinez was taking batting practice in a cage at the Rangers’ ballpark in Arlington. Looking on was Astros hitting coach John Mallee. Martinez noticed that Mallee was quiet. “How’d it look?” Martinez asked Mallee, when he was finished.

“Looked good,” Mallee said unconvincingly.

“Dude,” Martinez said, “what do you want?”

Mallee beckoned Martinez to sit next to him. “J.D., you’re not even a career .700 OPS hitter,” he said. “You don’t steal bags. You’re not a Gold Glover. You have to hit. Your numbers are O.K. You can have a career. You can bounce between the big leagues and the minors. You can make enough money to live off of, at least until you become too expensive to keep around. But that’s it. Unless you change something.”

Martinez was taken aback. “You’re asking me to change my swing, to change my way of making money?” he said. “Are you coming with me when they send me to Triple A after I go 0-for-20 because I’m trying to figure out a new way to hit the ball? No. You’re going to stay right here—and I’m going to be screwed.”

Still, Mallee’s message stuck with him. A few weeks later Martinez sprained his left wrist while sliding into second base in Toronto. He began spending most of his time in the trainer’s room, which had a TV that was always tuned to ESPN. The channel kept showing highlights of Brewers slugger Ryan Braun: He had been suspended for the remainder of the season for using performance-enhancing drugs. As Martinez watched clips of Braun, a five-time All-Star with an on-base-plus-slugging percentage that was more than 35% higher than his own, hitting home run after home run, he realized something: My swing doesn’t look anything like his.

Martinez called up clips of the game’s other leading power hitters, like Miguel Cabrera and Albert Pujols, and saw that their strokes were similar to Braun’s, which meant theirs didn’t look like his either. Then he began focusing on one of his teammates, catcher Jason Castro, who had just made the All-Star team and was on his way to tripling his career high in home runs. Holy crap, Martinez thought, Castro’s got the same swing they do.

While Martinez chopped down at the ball, the sluggers he studied were swinging with a smooth uppercut. The head of his bat finished low. Theirs finished high. He asked Castro how he had learned to swing like that. “You’ve got to see my guys in California,” Castro said.

The day after the season ended, Martinez had his first appointment with Castro’s guys in California: Craig Wallenbrock, who was in his late 60s, and his 27-year-old assistant, Robert Van Scoyoc. Neither Wallenbrock nor Van Scoyoc played past college ball, but they tutored players in a warehouse in an industrial park in Santa Clarita and had earned reputations as gurus of a modern approach to hitting. “I want to finish my swing the way Jason Castro does,” Martinez told them. He noticed an inscription on one of the many bats that hung from a rack on one of batting cage’s walls. “Thanks for everything,” it read. “I wouldn’t be where I’m at today without your help.” Below that was Ryan Braun’s signature.

Martinez was dedicated to self-improvement—a belief instilled in him by his parents. He had grown up in South Florida with five older sisters, all from his parents’ previous marriages. His mother, Mayra, was a nurse, and before another overnight shift at the hospital she would often drop J.D. off at the Domino’s where his father, Julio, worked. J.D. spent most of his grade school years popping slices of pepperoni into his mouth while Julio flipped pizzas. Julio eventually started his own roofing company, which grew until he had 35 employees. So when Wallenbrock and Van Scoyoc told Martinez that his swing was the worst they’d seen from a major leaguer, he viewed it not as an insult, but as an opportunity. He had made the big leagues, and stayed there, with a garbage swing, one that he could now hardly bring himself to watch. How did I get this far with that swing? he asked himself. How good might I get if I learn to swing properly?

The problem wasn’t just the way he finished his swing. That was bad, but it resulted from everything he did that led to it. Martinez started with his hands low and, as a pitch arrived, raised them. Braun and Castro kept their hands quiet. Martinez tended to lurch toward the ball. Braun and Castro stayed balanced. In batting practice, Martinez’s goal was to drive the ball to the back of the cage. Braun and Castro targeted the cage’s roof, aiming for an elevated launch angle that would be more likely to produce extra-base hits. Martinez spent five hours a day for two weeks in Wallenbrock and Van Scoyoc’s warehouse, revamping his stroke and consuming video that documented his rapid improvement. “It was like I was hitting in a room that was dark, and somebody turned the lights on,” he says. He went to Venezuela, a winter option that was rarely embraced by established big league players, to confirm that his improvements were real.

Officials with the Lions had one piece of advice for players staying at the team hotel. “You’re safe here,” they said. “But don’t ever, ever, ever go outside.” Caracas was soon to pass San Pedro Sula, Honduras, as the most violent city in the world. Martinez had no intention of sightseeing. “All I did was watch video and pray the Wi-Fi was working so I could watch more video,” he says.

When he arrived at Estadio Universitario, he encountered an environment different from anywhere else he had previously played. For rivalry games, the first row of the outfield bleachers was occupied by police with riot shields. The stadium held fewer than 21,000 fans, but during games they sounded like 100,000. They chanted, Ponche! Ponche!—“Strikeout! Strikeout!”—whenever a batter had two strikes, even if he happened to play for the home team. I’m not ponche-ing, Martinez said to himself. No way.

In his first game with his new swing, Martinez hit two home runs. He would hit six during his month in Venezuela, batting .312 with an OPS of .957. Better than the numbers was the way he felt when he hit the ball. “The first time, it was like, What just happened?” he says. “It felt like I was cheating. Like the ball was stuck to my barrel and I could do whatever I wanted with it.”

When he arrived in Kissimmee, Fla., for spring training in 2014, he sat down with Luhnow and Bo Porter, who was entering his second season as manager. “I’m not the guy I was before,” he told them. “I went down to Venezuela, and I discovered this new thing. Anything you want me to do, I’ll do it. Just give me the same at bats as anyone else. Let me show you what I learned.”

He thought they had agreed. They hadn’t. The algorithms developed by Sig Mejdal, a former blackjack dealer and NASA rocket scientist who was Luhnow’s chief data man, indicated that the probability that a player who was already 26, and who couldn’t run or field particularly well, would get better was vanishingly small. And Porter, eager to move ahead with younger outfielders, had little interest in giving Martinez the opportunity to demonstrate his new swing. He got just 18 at bats in the spring, half in pinch-hit appearances that prevented him from continuing the rhythm he had begun in Venezuela. “That’s not an opportunity,” Martinez says. “That’s an opportunity to fail.”

In his 18 at-bats, he had three hits. Near the end of spring training, Luhnow called him into his office. The Astros were releasing him. “I was in shock,” Martinez says. “Dude, I just got cut from the worst team in baseball.” Then he got mad.

“What happened?” he asked Luhnow. “I’ve always been cool. I’ve always been respectful. What did I do? What went wrong? When I asked for an opportunity to show what I’d learned, you said I’d get one. I only got 18 at bats.”

By the time Luhnow finished explaining that they didn’t envision him making the team, and that they wanted to go somewhere that he might, Martinez’s thoughts had turned to the future. As he packed his belongings in the clubhouse, longtime teammates José Altuve and Dallas Keuchel came by his locker to shake his hand. “I’m sorry, bro,” they said. “Keep your head up.”

“You guys are going to see me,” Martinez said. “Don’t worry about it. I’ll be good. I promise you.”

Two days later the Tigers signed Martinez, and he made the hour drive from Kissimmee to Detroit’s spring camp, in Lakeland. A few days after that, he was back in Kissimmee to play in a minor league spring game. Luhnow watched from the stands, as did several of Martinez’s old teammates. Martinez went 3-for-4, with a home run and five RBIs. After his homer, he tossed his bat as far as he could. The next day, the two teams played again, this time in Lakeland. Martinez hit three home runs—which would have been four if his last shot hadn’t hit the top of the wall. After the third blast, the Astros’ shortstop, a prospect named Carlos Correa, playfully threw his glove at Martinez as he trotted past him. “Get out of here, bro,” he said. “Who are you?”

Luhnow knew who he was. Martinez was the new hitter he had promised he had become, and who hadn’t had an opportunity to show it. What did we just do? thought Luhnow.

The Astros had released, for nothing in return, the very thing that their rebuilding process had been designed to find: a cheap superstar, and one whose contract they would have controlled for the next four seasons. After hitting 10 home runs in 17 games with the Tigers’ Triple A affiliate in Toledo, Martinez debuted with Detroit in late April. In mid-June, he was named the AL Player of the Week after a seven-game stretch in which he batted .444, with four homers and 11 RBIs. Shortly thereafter, he returned to Houston for the first time as a visiting player. When he finished taking batting practice, he found a familiar face waiting for him on the dugout steps. “I just want to tell you a few things,” Luhnow said. “I want to tell you congrats. I want to tell you that I wish you the best, and that I’m really happy for you. And I want to tell you that you were right. But take it easy on us.”

“Thanks, I truly appreciate that,” Martinez said. “And I won’t.” Over the next four years, which he began with the Tigers and finished with the Diamondbacks, Martinez batted an even .300 and averaged 32 home runs. His OPS of .936 was nearly 100 points better than Ryan Braun’s.

Martinez acknowledges that every athlete who has ever been cut feels aggrieved. What hurt most was how the Astros had done it. “They had all this data, all these nerds and geeks, and I think they forgot that at the end of the day, everybody is still human,” he says. “And a human can adapt, and a human can adjust.”

In the years to follow, J.D. Martinez would serve as a reminder of a few things for the Astros – an inescapable one, as their executives rarely flipped on their TVs without seeing him hit another homer. One, as Sig Mejdal said: “The feeling that we are smart is our enemy. That’s what we strive to avoid. SportsCenter reminds us of that every couple of days.”

Another reminder was not to overreact to even humiliating setbacks, but to use them to evolve. When a player was adamant that he had made a change over the off-season, the Astros committed to gathering enough information to determine whether it was meaningful one. They did it, in part, by investing in a technology called Blast Motion, a disc containing an accelerometer and gyroscope that is attached to a player’s bat and measures its velocity and angle as it moves through space. If Martinez had one of the sensors stuck to his bat’s knob during spring training of 2014, the club’s coaches and executives wouldn’t have had to rely on just his word and a small sample of plate appearances to decide whether his revamped swing was real. They would have seen it.

“Nine out of ten times, when people tell you they’ve gotten better in winter ball, it turns out it’s not true,” Luhnow said. “Sometimes it’s actually real. I am so happy for J.D. I give him a big hug every time I see him. I think about what could have been. And I also feel disappointed that he didn’t get more playing time to show us the new him.” Luhnow’s disappointment had already revealed itself back in 2014. He fired Bo Porter and replaced him with A.J. Hinch, the manager who would—even without Martinez—go on to lead the Astros to their first ever World Series title last fall.

This season, finally, Martinez has gotten over his grudge against the Astros. “They did me a favor by cutting me—they really did,” he says. “I don’t really care about that anymore.”

Now he expends most of his mental energy on hitting, not just preserving his stroke but also helping his new, largely younger teammates take better cuts. Last year’s Red Sox hit just 168 home runs, fewest in the AL, but this season they’re on pace for 216, despite the presence of few new faces aside from Martinez’s.

The power boost has something to do with the aggressive philosophy espoused by Cora and hitting coach Tim Hyers. Cora was Houston’s bench coach last year, and the scouting report on the Red Sox—one the Astros exploited during their four-game victory in the ALDS—was simple: They take too many pitches, so it’s easy to get ahead of them in the count. On Jan. 4, Cora sat down inside the Hotel Commonwealth, just over the Massachusetts Turnpike from the Green Monster. Sitting across from him was Mookie Betts, the 25-year-old outfielder who had nearly won the AL MVP award in 2016 but whose production had modestly tailed off last year, along with most of his team’s.

“You’re going to lead off for us this year, and the first pitch of the season is going to be a fastball right down the middle,” Cora told Betts. “You’re going to swing at it, and you’re going to hit it out of the ballpark.” Three months later, leading off on Opening Day, Betts did get a first pitch fastball from the Rays’ Chris Archer, and he swung at it. While the ball fell just short of a home run—Tampa Bay centerfielder Kevin Kiermaier tracked it down at the base of the wall in Tropicana Field—the at-bat set the tone for all that was to follow. In 2017, Boston hitters swung at just 62.3% of strikes, the lowest rate in the league. Now they’re at 68.8%, the seventh highest.

But the Red Sox attribute their offensive surge to Martinez, too, and not just because of the homers he hits himself. In Detroit, Martinez’s teammates often mocked him for his strange bag of trinkets, but in Boston they use them, under his instruction. “I’m kind of like a third hitting coach,” he says, after Hyers and Hyers’s assistant, Andy Barkett. It is not unusual to find Betts—again an MVP candidate, alongside Martinez, and one who is on pace for a career high 37 homers—in the batting cage beneath Fenway with a kickball pinned under his left elbow, and Martinez whispering in his ear. They began working together near the end of spring training, and soon Betts was sensing the results. “I can’t keep the ball in the yard right now, bro!” Betts told his new teammate. “I’ve never done anything like this before.” It was a feeling that Martinez by then knew well, one he’d first experienced four years earlier, in a stadium ringed by police holding riot shields.

Martinez was late to arrive in Fort Myers this spring, because the Astros proved not the only team to undervalue him. He was a free agent last winter, the top talent on the market after batting .303 with 45 homers, but he lingered without a buyer until late February when the Red Sox signed him to a five-year, $110 million contract—much less than he felt he was worth. “I guess I don’t have the flashy talent other guys have,” he says. “I’m not Mike Trout or Bryce Harper or Mookie Betts. I’m not going to blow you off the field. But in the box, I know what I’m doing.”

And so J.D. Martinez’s motivation now is to demonstrate that underestimating him was not just the greatest mistake made by one team, the Astros. It was the greatest mistake made by 29 of them.