When the Boston Celtics arrived at Washington's Capital One Arena to take on the Wizards for a matinee on Jan. 23, there was no indication it would serve as the turning point of Boston's tumultuous season.

The Celtics had been coming off one of their most dispiriting losses of the season, blowing a double-digit, fourth-quarter lead at home to the Portland Trail Blazers, who were without star point guard Damian Lillard.

Jayson Tatum had missed 18 consecutive 3-point shots over a four-game stretch. The team's offense had hovered around the NBA's bottom 10. And the Celtics had fallen to 23-24 and 10th place in the Eastern Conference -- a half-game ahead of the New York Knicks for the final spot in the play-in tournament.

More than halfway through this season, Boston looked headed for a brief stint in the playoffs -- if it made it there at all.

Then came the 116-87 demolition of the Wizards, which saw Tatum score 51 and lift Boston back to .500 on the season. In the locker room afterward, Marcus Smart sensed change was afoot.

"After that game, we just had this mentality and mindset and this sense of urgency that we can feel that a change was starting," Smart says. "Once that got rolling, and we got on the right track, it was just smooth sailing from there."

Since that moment, the Celtics have transformed into arguably the NBA's best team, finishing the regular season with the league's top record (28-7), offense (120.2 points per 100 possessions) and defense (104.8).

And, after sweeping Kevin Durant, Kyrie Irving and the Brooklyn Nets in the first round of the playoffs, Boston has a chance to tie its East semifinal showdown with the defending champion Milwaukee Bucks with a road win in Game 4 on Monday (7:30 p.m. ET, TNT).

But one of the more remarkable midseason turnarounds in NBA history didn't actually begin on Jan. 23. It began long before the results showed on the court.

"I don't think," Celtics president of basketball operations Brad Stevens said, "that anybody would have guessed that it was gonna take off like it did."

How Boston discovers its blueprint

The Celtics' low point came on Nov. 1, just seven games into the season. After blowing a 14-point lead to the Chicago Bulls, who outscored the Celtics 39-11 in the final frame, Smart called out Boston's superstar wing tandem.

"Every team knows we're trying to go to Jayson and Jaylen," Smart told reporters postgame. "Every team is programmed and studied to stop Jayson and Jaylen. I think everybody's scouting report is to make those guys pass the ball. They don't want to pass the ball."

The moment was a mile-marker on a campaign -- helmed by a trio of talented perimeter players that have taken part in well north of 50 playoff games for Boston -- that seemed destined to disappoint for a second straight season.

Instead, it became a distant reminder of the turnaround -- and what eventually started clicking for Boston.

"We have a lot of great guys on the team, so it was just a matter of figuring out how to play together," Grant Williams says. "Same thing with Ime [Udoka, who is a] first-year head coach, so all just learning how to establish ourselves not only in the league, but also establishing the blueprint the Celtics want to have.

"As we've grown, we know exactly what we want and exactly what we are."

Some of the Celtics' early woes were out of their control. Brown and Al Horford missed chunks of training camp, and Horford missed time in December after he entered the league's health and safety protocols. Smart, for his part, entered protocols in January and missed six games.

Boston also reversed its poor shooting luck. While Boston was 16th in the league in Second Spectrum's quantified shot probability through Jan. 22, it showed up in some particularly ugly losses, like when the Celtics had the single worst catch-and-shoot performance in Second Spectrum's history (since 2013) in a loss to the LA Clippers in December. Boston leapt to second in the league in quantified shot probability beginning with that win in Washington through the regular season.

The Celtics also found better fits for Udoka's preferred system.

The franchise had used part of its trade exception to land Josh Richardson in the offseason, and convinced Dennis Schroder to take a massive pay cut to play a role off the bench. Both, however, are used to holding the ball, which played right into the natural tendencies of both Tatum and Brown that Udoka was hoping to change.

Schroder and Richardson were dealt at the deadline -- Schroder, Enes Freedom and Bruno Fernando for Daniel Theis and Richardson and Romeo Langford for Derrick White.

The moves paid immediate dividends, particularly in helping Udoka implement the free-flowing offense he's trumpeted since arriving last summer.

Before the trade deadline, Boston ranked 19th in the NBA in assists per game, and 49.4% of their field goals came off passes, which ranked 18th. After it, Boston rose to seventh in both categories.

"If I could have picked the guy who would have been the perfect guy to come in and complement our group, it's [White]," Udoka says.

"He's a better offensive player than J-Rich, and a much better defender than Dennis, so you kind of get those guys combined into one."

Boston's starting lineup of Smart, Brown, Tatum, Horford and Robert Williams III had been elite. By bringing in White, and making more time for Grant Williams and Payton Pritchard off the bench, the Celtics had an airtight eight-man rotation that fit Udoka's game plan.

"Early on in the year, we had guys trying to figure out new roles," Smart says. "We experimented with new roles, new guys, new faces. It was a lot. [Now] the connectivity is showing."

Tatum ascends, and Smart finds his true role

It's easy to talk about ephemeral things like connectivity, teamwork and togetherness. But none of those will win games if they're not paired with talent.

"The biggest thing to happen in the NBA the last two months," an Eastern Conference executive said recently, "was Jayson Tatum becoming the player the world thought he already was."

Even after some especially rough shooting over the first half, Tatum finished the season averaging 26.9 points, 8.0 rebounds and 4.4 assists. More importantly, though, was his growth as a passer and playmaker over the season's second half.

His assist numbers shot up by nearly one per game after the All-Star break, and even earlier in the season, Udoka leaned into Tatum as the primary ball handler when Smart and Brown left the court. Boston outscored opponents by 12.2 points per 100 possessions with him on the court, and Smart and Brown off.

"The numbers are off the chart with him as a handler," Udoka said. "When you see what he is -- with his size, and vision and all that -- to me it's a no-brainer."

The final piece of Boston's transformation has been Smart taking over as the team's starting point guard. It's a role Smart has, for years, had begged to play.

"I'm just happy that I finally get to be put in a position I'm great at," Smart says. "Not saying that the other positions I was in, I wasn't good. It was just ... I was like a Swiss Army knife, and it was like I had no true role on the team. I was just doing a little bit of everything."

Despite being drafted in 2014 as a point guard out of Oklahoma State, he has played alongside a series of other point guards, from Isaiah Thomas to Terry Rozier to Kyrie Irving to Kemba Walker. But once Walker was traded for Horford last summer, there was no choice but to hand the keys to Smart.

Udoka, though, believed Smart could be just the player he needed to run his offense. He said that unlike other guards who have played with Tatum and Brown over the years, Smart is a pass-first floor general.

"Marcus will do one of his crazy-ass passes, and he's like, 'I got you,'" Udoka says, raising his hand to mimic Smart apologizing for a bad play. "It's like a Manu [Ginobili] situation with [Spurs coach Gregg Popovich].

"You gotta let him be who he is and live with some of the craziness ... what you do is trust Marcus."

A great defense becomes elite

Even when the Celtics were flailing in the opening months of the season, they ranked fifth in defensive rating. And, after trading for White and moving on from Schroder, Boston's top seven players -- Smart, Brown, Tatum, Horford, Robert Williams III, White and Grant Williams -- are all plus defenders.

And, with all of them standing at least 6-foot-4, and two of them (Horford and Williams) being centers, the Celtics can expect to have a size advantage at virtually every position when any combination of those seven are on the court together.

That translated to Boston being 4.6 points per 100 possessions better defensively than the second-place Miami Heat from Jan. 23 through the end of the season -- the same margin between the Heat and the Minnesota Timberwolves in 18th.

"Suffocating," Grant Williams said, when asked to describe Boston's defense.

"If you ask the players on other teams, that's how they would describe it, especially when everyone is locked in, everyone is doing their own job. ... Making teams play against us in the half court. That's where our strength is."

The Celtics spent the first round making life miserable for Durant, holding him under 40% shooting for a series for the third time in his career. They forced Giannis Antetokounmpo into 18-for-52 shooting in the first two games of the East semifinals. Any time Boston talks about its defense, physicality is repeatedly mentioned as a core tenet -- and the Celtics have applied that with gusto in these playoffs.

It's a far cry from how Boston looked when it was steamrolled in five games by the Nets in the first round last year.

"This is probably the best defensive team that I've been on," White said. "Just having guys out there that are connected, can guard multiple positions. That's just the way the NBA is nowadays. You need guys to be able to do that."

It's a massive aspect of the formula that has Boston still dreaming of lifting championship banner No. 18 to the TD Garden rafters.

"I hope that everybody's really enjoying competing together because I think that we have a group that is moving in one direction -- no drama," Stevens says, "and we got to keep it that way.

"I know how fragile it is. ... so I believe that when the brightest lights are on, we're gonna be hard to beat."