Halak made 41 saves, Marc-Andre Bergeron scored a 4-on-3 goal in the last 30 seconds of the opening period Wednesday night, and eighth-seeded Montreal held on to beat Washington 2-1 in Game 7, stunning the Presidents' Trophy winners by reeling off three consecutive victories.
The Canadiens are the ninth No. 8 team to knock off a No. 1 in 32 matchups since the NHL went to its current playoff format in 1994 - and the first to come back from a 3-1 series deficit.
Dominic Moore made it 2-0 for the Canadiens with 3 1/2 minutes left in the third period, stealing the puck from defenseman Mike Green and beating goalie Semyon Varlamov. That silenced the home crowd - but the Capitals didn't go quietly.
Brooks Laich cut Washington's deficit to a goal by poking home a shot while down on his knees after Ovechkin put the puck on net off a rebound. That made it 2-1 with 2:16 left, and the Capitals kept pushing frantically forward, pulling their goalie for an extra attacker down the stretch.
A high-sticking penalty on Montreal defenseman Ryan O'Byrne with 1:44 to go put the Capitals on the power play, which they made a 6-on-4 skating edge with Varlamov on the bench. But Halak was up to the task every time. His teammates piled on him at the final horn, while Ovechkin skated all alone toward the other end of the ice, his stick resting on his knees, and his head bowed.
The league's two-time MVP has played in four career playoff series, and each has gone to a Game 7. The Capitals are 1-3 in those deciding contests.
Montreal, meanwhile, moves on to face Sidney Crosby and the defending Stanley Cup champion Penguins in the second round, with Game 1 at Pittsburgh on Friday. The fourth-seeded Penguins were the only higher-seeded team to win a first-round series in the Eastern Conference.
Yes, that's right: All three division champions in the East are done already - Washington, New Jersey and Buffalo.
All in all, this series represents a monumental collapse by Washington, which blew its big lead after entering the playoffs with such high hopes thanks to earning a third straight Southeast Division title, compiling the league's best record for the first time and leading the NHL in goals.
This is also a club that preceded each home game against Montreal with a video display that included an image of the Stanley Cup and the words, "NOTHING ELSE MATTERS," while the Metallica song of that name blared.
The Capitals owned the best home record in the NHL during the regular season, but lost Games 1, 5 and 7 in Washington.
Montreal allowed more goals than it scored this season and only qualified for the playoffs on the final weekend, sneaking in with the worst record of any of the 16 teams in the postseason.
But they had Halak on their side.
He was yanked during Game 3 in favor of Carey Price, who also started Game 4. But Montreal coach Jacques Martin went back to Halak for Game 5 - a switch that worked out rather well.
Halak made 37 saves in a 2-1 win in Game 4, 53 saves in a masterpiece of a 4-1 victory in Game 5, and then produced more of the same Wednesday. All told, Halak stopped 131 of 134 shots in the last three games.
And it was fitting that the series ended with an unsuccessful Capitals power play. They led the league in that category in the regular season by converting on 25 percent of man-advantage chances, but went only 1 for 33 against Montreal.
In Game 7, the Capitals dominated the opening period for stretches, keeping the pressure on Halak. But it was the Canadiens who led 1-0 at the first intermission, thanks to Bergeron's goal with 29.1 seconds to go.
Bergeron - a defenseman on the ice for only 1:25 in the period, less than any other player - took a cross-ice pass from Scott Gomez and, all alone at the top of the right circle, let fly a slap shot as he dropped to a knee.
The goal came barely more than 10 seconds into a 4-on-3 that was set up by a cross-checking penalty on Green, a finalist for the Norris Trophy as the league's top defenseman. Roughly 1 1/2 minutes before Green went to the box, Washington's Brendan Morrison and Montreal's Tomas Plekanec both were whistled for roughing during a scrum near Halak.
Holding that lead, the Canadiens were often content to focus on defense, and they recorded only three shots in the second period. Overall, Montreal was outshot 42-16.
Bergeron's goal quieted a crowd that had been rowdy in the early going, breaking into raucous chants of "Let's go, Caps!" before and after the national anthems. Disappointed home fans even let loose scattered boos after Ovechkin lost the puck while trying to bring it back into the offensive zone up the right wing during a power play.
All of 24 seconds into the third period, Ovechkin put the puck past Halak on a shot from the left circle, but the goal immediately was waved off by an official because Washington forward Mike Knuble was in the crease, backing into Halak.
The Capitals - and their red-clad, flag-waving fans - did not realize at first the apparent goal did not count. Ovechkin did his customary slamming into the glass to celebrate, and his teammates mobbed him, while the crowd roared. But the call was announced, leaving Capitals coach Bruce Boudreau leaning over the boards, gesticulating and yelling, while spectators yelled full-throated insults at the official.
Later in the third period, Montreal's Maxim Lapierre slammed into Varlamov in the crease, forcing the goalie and the puck into the net. There was no goal - and, to Washington's frustration, no penalty, either.
NOTES: Washington's Alexander Semin, who scored 40 goals this season, whiffed on a great chance in the first period and finished the series without a goal. ... Bergeron's goal was his first in the playoffs since 2007, when he was with the Islanders. ... It's only the second time Montreal has won a series after trailing 3-1.
Round 2 schedule