The Boston Celtics practice facility is tucked in the back of a high-end private gym in Waltham, Massachusetts. You walk in — past front desk personnel wearing broad grins and trim polos, girls in yoga pants, guys in bandannas, grandmas on treadmills — until you reach a windowless room. It’s large, drably carpeted, and sparsely outfitted: There’s a pull-down screen and a projector; hard, uninviting chairs; and a lone carton of individually wrapped Twizzlers. And what reminds you that you are not, in fact, currently attending an SEO optimization seminar in Conference Room B of the Downtown Cincinnati Ramada Inn are the glass display panels full of Celtics history.

The memorabilia moves chronologically, from right to left. There are black-and-white roster portraits of the early Celtics champions, action shots of Hondo and Dave Cowens, scraps from Johnny Most’s retirement ceremony. There’s the starting five — Bird, D.J., Ainge, McHale, and Parish — in an astoundingly ’80s poster titled “A Portrait of Excellence” and a life-size illustration of Larry Bird bending at the free throw line.

In the next section, the display skips to 2008. The centerpiece is a giant photo taken after the Celtics beat the Lakers to claim the franchise’s 17th title, the whole team trying to pose in orderly rows but sloppy and cracking up and falling all over one another instead. That’s bordered by a photo of Doc Rivers getting splashed with a torrent of orange Gatorade, and another of Rajon Rondo, on a Duck Tour boat, alone. And everywhere, there’s Ray Allen, Paul Pierce, and Kevin Garnett, the former Big Three: on the cover of ESPN The Magazine, kibitzing with Bill Russell, grinning and squinting through champagne (or Bud Light?) in the locker room.

In July, Allen signed with the Miami Heat, the team that ended the Celtics’ past two seasons, and the team they’re targeting this season. He did so for less money than Boston offered him, and he did so while wagging an accusatory finger at the Celtics organization and its surly genius point guard. When he comes back to Boston, Allen has told fans, “Don’t boo me, boo the [Celtics].” During the offseason, Rondo would refer to Allen only as “that guy.” KG says he’s lost his former teammate’s phone number. But it’s not like they can actually forget him. For one, he’s right there, every day, in the memorabilia room.

With Allen gone, the Big Three era has officially reached its quiet and contentious end. So why does the mood feel so peppy in Boston? In large part, it’s because of the guy who signed with the Celtics a week before Ray split. His name is Jason Terry, he’s got a lovely jump shot of his own, and he seems pretty goddamn sure that the C’s are winning it all this year.

The first tattoo Jason Terry shows me isn’t the fresh Leprechaun on his right biceps, but the old Underdog on his left shoulder. We’re sitting on a cushioned bench in a corner of the practice court, a few minutes after scrimmage has wrapped. I ask Terry about his new team. There are, it seems, a good amount of long-shot prospects: Jeff Green, who was sidelined last year with a scary heart condition; first-round draft pick Jared Sullinger, whose medically flagged back prevents him from fully extending his legs in a seated position; and Terry himself, who has to be the difference for a team most basketball experts consider little more than a hurdle in the Heat’s path to a third consecutive NBA Finals appearance.

“A lot has to break in your favor,” I suggest. “Kind of an underdog situation in general, huh?” Terry smiles, then pulls up his sleeve. “I’ve always been the smallest guy on the court,” he says, rubbing Underdog. “That’s what it’s been for me my entire career.”

Terry had other suitors in free agency, like the Memphis Grizzlies, but Boston made the best offer (three years at $15 million) and Boston had the banners. Doc’s pitch to Terry was all about the banners. Pierce’s was too: “He was like, Man, you should have been on our team three years ago! We would have had three championships by now.” Garnett’s, expectedly, was way weirder. “I heard from KG first,” Terry says. “His phone call was simple. It was, ‘You love green, you’re from the Emerald City [Terry grew up in Seattle], and green is your favorite color. So why not be a Celtic?'”

After the Mavs lost to the Heat in the 2006 Finals, Terry says, beating Miami in 2011 wasn’t pure joy: “It was more a huge sigh of relief. It was like a weight had been lifted.” And he still hasn’t gotten over what happened last season, when Dallas management didn’t bring back the same team to compete. It left him craving championship-level basketball. “Now that I’m back in the position to do it again,” Terry says, “I’m just as hungry as these guys — being one game away from the Finals.”

That ‘one game’ was Game 7 of the Eastern Conference finals, when the Heat ran away from the Celtics in the fourth quarter. Terry says he has watched that game “10 times since I’ve been in Boston”; Rivers has run the footage on a loop in the locker room. Terry was watching live, too, and, he swears, “For some reason I just kind of imagined — what if I was out there? What if I was there playing with the Celtics against Miami?”

Now it’s time to flash the biceps. In 2010, before the season, by way of putting lifelong body art where his mouth was, Terry got a tattoo of the NBA championship trophy. To most, it was a joke — and then the Mavs won it all. This year he outdid himself.

“I came in early,” he says, by way of explanation. “I’ve been here since September 4. I’d come in at night and get tons of shots up. And just coming in here and looking at all the banners — it makes your blood boil. I was just like, ‘Man, with this team — I know we gonna win it. I just know.” So, while celebrating his 10-year wedding anniversary with his wife in Hawaii, Terry made a pit stop at a tattoo parlor. “I was like, ‘Yeah, let me go ahead and get this Celtics guy tattooed on.'” He pulls back the other sleeve now, flexing Lucky the Leprechaun, who had been planted beneath his former predictive ink. “And as you can see, he’s spinning the Larry O’Brien trophy.”

He continues: “I’m not a psychic. I’m not a prophet. But I’m a big faith-fearing guy, and in my faith, sometimes, you have to speak what you believe, and it will come true. So, yeah, I have no hesitation in telling you that our mission is to kill whoever we step on the court against.”

What do the other guys think of the tat? “They like it,” he says. “They say if we win, they ‘gon do it too.”

Everybody likes to feel wanted.

I’ve been a Celtics fan since I was 10, when my family moved to Massachusetts. I only had a vague idea what basketball even was before that. We lived in Israel, and then the Netherlands, where in gym class we played cricket. And so everything about the Celtics’ long, proud history felt more or less fictional. When I got to Boston the big names were Dee Brown, Rick Fox, Dino Radja, and the fading spectres of Xavier McDaniel and Dominique Wilkins.

And I loved them! I loved them so much. One of my clearest memories from our first year in the country was watching a Celtics-Heat game from the balcony at the old Boston Garden. Seconds left, the Celts down one, and they feed Dino underneath — he crams it for the win. I had my eyes squeezed shut during that entire possession, too scared to watch. I opened them with the roar of the crowd. The next year they tore the Garden down.

We don’t need to run through the mostly sad history of the team between then and The Big Three. This is just to say that, since I fell for the Celtics after all the good stuff had happened, I never associated the team with winning. And so when they became an elite team — rationally or not — it always felt tenuous.

I think it’s fair to say that Jason Terry is the biggest name the Celtics have acquired via free agency since I’ve been a fan. And to know Jet wanted to come here, just as Ray Allen was making it a point to leave? Well — not to get too touchy-feely here, Jason — but it meant a lot, bro.

Meanwhile, the Ray situation gets uglier. It started with anonymously sourced jabs; now he’s just unloading. In a Miami radio interview, Allen said, “When this contract situation came down, everybody in my circle — mom, family, brother, sister, friends from college, people who watched me since I was in high school and since I was in college — nobody wanted me to re-sign in that situation because they thought, There [is] so much left in you and this team isn’t taking care of you or treating you right.” And while reports originally had Ray chafing at Rondo, Allen took pains to make it sound like it was the other way around: “I had no issues with him. I won with him … [if Rondo] had issue with me, that’s on him.”

The original reading of Allen’s departure was “gut punch.” Quickly, though, it’s become something else.

It’s hard to believe that the contract Boston offered Allen — two years, $12 million, compared to the three years, $9 million he got from Miami — was, as Allen appears to believe, a hollow offer. But it may work out for Boston in the end. The rift between Allen and Rondo was unfixable, and if you’re Danny Ainge, surely you’re riding with the Young Surly Genius over the 37-year-old with bad ankles. Basketballwise, both Doc and the impartial experts concur: Between Terry, fellow free-agent signees Courtney Lee and Leandro Barbosa, and Avery Bradley — whose late-season emergence marginalized Allen in the first place — the Celtics’ backcourt rotation has clearly improved from last year.

But, for now, forget about “actual facts” and “concrete conclusions” about how this team has improved. Let’s get to the warm emotional goo of team chemistry.

This preseason the Celtics played in Istanbul and Milan, a move that consciously recalled the team’s trip to Rome before their championship season. According to Terry, the team would hang “every night. And every night it was everybody — no man left behind.” After dinner, they’d hang out, talking in the lobby for hours and hours. “I mean, I’m talking about these talks went till two, three in the morning,” he said. “Playing cards, telling old war stories.” Terry says KG had the best tales, “from battles against old veterans like Charles Oakley to gang shootouts when he was younger.”

Earlier in the offseason, Rondo organized unofficial workouts for the core group in L.A. Terry explains: “I boarded the flight that he paid for. Stayed for a week. It was almost like practice, ’cause he ran it. We ran through the plays in the morning, and then we scrimmaged. At night we’d go to dinner and then we’d get right back in the morning and do the same thing. The last day was special. He tricked us. He said, ‘Look we gon’ go outside for a minute.’ We go outside and we break out into a flag football game.”

The Celtics beat reporters tell me it’s like night and day with last year: Rondo’s one-syllable answers have been stretching to heretofore unimaginable lengths. And now here comes happy-go-lucky Jason Terry, getting inked, promising titles, and continuing to exhume all manner of bad vibes by saying stuff like, “We’re gonna go right at it, head-on. Obviously [Allen] didn’t want to be a part of what we’re building here. So he’s the enemy,” and, “I’m gonna do what I do — be the Jet. Fly at the highest level possible. Cruising altitude.”

Then again, when I ask Rondo what the difference between playing with Ray and playing with Jet is — on the court, off the court — I get a sharp, curt answer: “I don’t compare ’em.”

The day after practice the team is in Hartford, Connecticut, for a preseason game against the Knicks. Paul Pierce is on the court with a rubber cord around his ankles, doing a stretch that makes it look like he’s pooping while walking sideways. In the locker room Brandon Bass has spotted a couple of beat reporters wearing identical dress shirts, and he’s giving them guff for it. Then he identifies their checkered print as gingham, and says, “I bet you five bucks Rondo doesn’t know it.” Strength-and-conditioning coach Bryan Doo is running around with thin wheat-bread sandwiches on plastic plates, shouting, “Anybody want a sandwich?”

With the game about to kick off, the arena is surprisingly loud. One Knicks fan in particular sticks out. He’s decked in vintage gear, he’s screaming his lungs off, and he’s insistently waving an airbrushed T-shirt bearing the image of Skip Bayless.

The action starts, and I know it’s preseason, but with the young and new talent performing, it’s hard not to see Tantalizing Glimpses of the Future! Jeff Green drops a nifty transition handoff to Sullinger, who scores breezily. Rondo streaks a pass up the court to Terry, who snaps it over to KG for the slam. Rondo tosses a mid-air behind-the-back pass to Green, who finishes with a ferocity that causes J.R. Smith and Raymond Felton to stifle reactions on the sideline.

The game ends up tight, and the starters get intense cheering on the backups. His knees already wrapped in ice, Terry hobbles out near the 3-point line during timeouts to flap his arms to the crowd. After the Knicks pull out the overtime win, Terry tosses a white Reebok to a woman in the front rows, and she squeals with delight. In the locker room, Terry takes the occasion to do some raving about his new point guard: “Sometimes you watch him in the middle of the game, and you can see him make a pass, and you almost lose focus — you’re just like, ‘Wow.'”

I ask Terry if he feels any increased pressure about the beginning of this new, post–Big Three Era. It’s the only time I see his aura of positivity blink even a bit. “Our sole mission is to win the championship this year,” he says. “We are not looking for the future. And that’s all we can do. I tip my hat off to them. They had a good run. They’ll go down as a threesome that was” — he pauses — “pretty good. But it’s a new day. And we’ll see what happens.”

You could argue that, objectively, the Big Three era of Garnett, Pierce, and Allen was marked by underachievement — one title, another Finals appearance, an ignominious end. Subjectively, I’d say it was a total goddamn blast.

New York Magazine has a long-running feature called the “undulating curve of shifting expectations,” a tongue-in-cheek way of charting pop-culture events against whiplashing expectations over time. The last five years in Boston were like the NBA version of that undulating curve.

In 2009, a year after winning the title, Garnett goes down for the playoffs, and instead of being a repeat contender, the team becomes a scrappy unit going after the big fish. (The epic first-round series against the Bulls, in which Ray Allen performed so admirably, never happens if KG is around.) The next year, Garnett returns, but a late-season swoon suggests the Celtics’ championship window won’t reopen. Then the Celtics tear through the playoffs, vanquish LeBron’s formidable Cleveland team, and, fwoomp, once again they’re contenders. But a heartbreaking Game 7 loss to the Lakers chops them back down. And just last year, as Jason Terry so vividly recalls, if the last six minutes of Boston’s season had played out differently, who knows how we’d be framing their 2012-13 chances.

I watched the Celtics win in 2008 in an East Village bar stuffed with Boston partisans. In the waning seconds, with the C’s up huge, a friend of mine told me: “Enjoy this one; there’s nothing like the first one.” I nodded, smug and content that this was the beginning of a dynasty. Maybe if the Celts had rattled off multiple championships I would have felt progressively less elated. Instead, they scrambled and fell and got back up, again and again, and every year they found new ways to give me heart palpitations. Now, here comes another season in which I have no idea what’s going to happen.

A couple days after Hartford, the Celtics are at the Barclays Center in Brooklyn for morning shootaround. The place is still so new that there are construction workers milling around, and drilling noises, and groups of guys in sharp suits pointing up emphatically while saying things like, “We need installations up there!” The Celtics roll in casually and scope out the fresh court. Rondo says the low lights in the arena make him feel like he’s onstage. Then he and Jet get to firing up jumpers while squabbling over a card game: “I won the first game.” “No, I won. I had a seven of diamonds.”

Before the media got booted from the closed practice, I watch with a security guard in a back tunnel. Turns out he’s a Celtics fan. “I’m Jamaican, we have different nicknames for everyone,” he says. “Garnett, I call him dogface. He be doing that dogface.” Then he growls, demonstrating what many of us might know as Garnett’s serial killer face. “When he’s out there, he’s like, ‘Eat that shit!” Then he does an exaggerated miming of a huge swat. We start talking about the playoffs two seasons ago, when Miami bounced Boston in five games. “It literally brought tears to my eyes,” he says.

After shootaround I catch Terry in front of the team bus, and we chat for a minute about what’s been going on outside of basketball. He’s been hitting up Sox and Pats games, getting all kinds of love from the Boston diehards: “Fans are running up to me like, ‘Ayo, the Jet! We’ve been waiting for you to land,'” he says. “Been to the school three or four times for parent night. Seen Big Papi at parent night — that was kind of cool. Been to the pumpkin patch. I’m all over, man; I’m just having a great time with it. And it looks like it might, if things work out, be my third home. If my prediction is true.” And here he again rolls up his sleeve, flashes his biceps, and pats his lucky Leprechaun.

This article has been updated to clarify the terms of Ray Allen’s Miami contract.