A day before the rest of the league is allowed to start the free-agency process, the Boston Celtics signed arguably the most desirable unrestricted free agent at well-below market value.

Somewhere, Danny Ainge has to be cackling in delight.

A summer of uncertainty became much more definitive when the Celtics agreed to a three-year contract extension with Kevin Garnett. Before you could even settle into your Fourth of July weekend, the Celtics sent up some fireworks of their own.

With Kevin Garnett rejoining Rajon Rondo next season, two key pieces of the Celtics' past success remain in the present. Jared Wickerham/Getty Images

More importantly, the deal sets into motion Boston's entire offseason. The new contract might as well come with a new nickname for Garnett: The Big Domino.

Oh sure, he's still the Big Ticket. This contract -- valued at $34 million over three years, sources confirmed -- will push Garnett's career earnings to a whopping $325 million. That's the most money pulled down by a player in league history, with KG leapfrogging old friend Shaquille O'Neal and his $292 million.

But even at age 36, Garnett was the linchpin to Boston's success this past season, and that's probably true looking ahead to next year as well. Ainge had noted that Garnett was Plan A. Truth be told, the Celtics really didn't have a Plan B, at least not one they preferred to execute.

But now Garnett is back, and so, too, are the Celtics. Yes, we're talking about potentially adding Years 6, 7 and 8 to what was supposed to be a three-year window with this core, but it's hard to argue with the results.

The Celtics had two chances to knock off the eventual NBA champions before succumbing to the Miami Heat in the Eastern Conference finals earlier this month. Despite all sorts of adversity faced during the 2011-12 season, the Celtics were 8 minutes short of winning Game 7 on the road.

This team genuinely believes that by bringing back its core, getting a healthy Avery Bradley -- maybe a healthy Jeff Green, too? -- and adding some pieces, Boston can make another charge at a title.

None of that was possible without Garnett.

Even as he pondered retirement, it was hard to believe Garnett would truly walk away. Yes, the wear and tear of 17 NBA seasons and the 12th-most minutes played in league history has taken its toll on a prep-to-pros player once dubbed "The Kid." Now Garnett is a geezer by NBA standards -- please don't tell him we said that -- but played as if he found a fountain of youth last season.

The Celtics needed Garnett back. He's the defensive anchor and the team's conscience. Paul Pierce might be the captain, but Garnett set the tenor when he arrived in Boston, and he holds everyone accountable.

It would be nice to see Ray Allen in a Celtics uniform again, but there's a chance he could opt to don a Heat jersey. Mike Ehrmann/Getty Images

Everything would have been different next season without Garnett on the floor or in the locker room. Now Boston doesn't have to worry about that.

But there's still work to be done. So where do the Celtics go from here?

With Garnett in the fold, the Celtics now have five players under contract for next season at a total of about $45 million: four starters in Rajon Rondo, Bradley, Pierce and Garnett, plus soon-to-be second-year forward JaJuan Johnson. Boston also extended a $1.05 million qualifying offer Friday to center Greg Stiemsma and added big men Jared Sullinger and Fab Melo through Thursday's draft.

That's eight players at roughly $48 million, and the team holds low-cost options on E'Twaun Moore and Sean Williams -- decisions that can wait a few weeks. The more pressing issue is the group of free agents the team owns varying rights to, including Ray Allen, Brandon Bass, Mickael Pietrus and Keyon Dooling.

With Garnett's situation resolved, it thrusts Allen into the spotlight. The Celtics can go over the salary cap to retain his services, allowing them to pay more than any other contender can offer. The question is whether Boston desires to do that. Allen moved to a reserve role last season, and the team has to put a value on his remaining skill set. He's still an offensive force and the league's all-time 3-point leader, but his defense can be a liability at times.

And this is why it's Ainge's next big decision: The Celtics will be above the league's roughly $58 million salary cap, but if they can stay within $4 million of the $70 million luxury tax, it would afford the team the ability to utilize both a $5 million midlevel exception and a $1.975 million bi-annual exception available to those teams under the threshold.

If the Celtics have to splurge on Allen and scale above the threshold, they'd only be able to offer the $3 million taxpayer exception, which it used last season on Chris Wilcox.

Don't discount the difference. If Boston stays under the threshold, it could pursue a big-name free agent like O.J. Mayo with the $5 million exception, then still have the bi-annual exception in order to re-sign someone like Pietrus or Dooling (or fend off anyone that submits an offer sheet on Stiemsma, now a restricted free agent).