The victory gave Boston a 3-2 lead in the best-of-seven Eastern Conference final, with a chance to clinch Wednesday night in Tampa Bay. The Lightning would need to win Game 6 to force a seventh game in Boston on Friday.
Nathan Horton scored to tie it in the second period, 17 seconds after returning from the penalty box. Marchand scored with 4:04 left in the period to give Boston a 2-1 lead.
Boston managed just 19 shots on Mike Smith as he made his first career playoff start in place of Dwayne Roloson.
Tampa Bay finally managed to pull the goalie with 42 seconds left, but Rich Peverley scored an empty-netter with 12.1 seconds left to clinch it. The Lightning left the extra skater on the bench for the ensuing faceoff, but they couldn't come through with anything more than some shoving after the final whistle.
Thomas bounced back after allowing four straight goals in Game 4 and may have saved the season when he stopped Steve Downie with about 11 minutes left, reaching out to put his stick in front of the open net and protect a 2-1 lead. The crowd cheered each time it was shown from a different angle on the scoreboard.
Two nights after Tampa Bay rallied with five straight goals to overcome a 3-0 deficit and win Game 4, Boston shrugged off Simon Gagne's goal just 69 seconds in and staged a comeback of its own.
Gagne, who scored the winner in Game 4, converted a 2-on-1 with Steven Stamkos to beat defenseman Johnny Boychuk and put the puck past Thomas. Horton tied it on a one-timer from Milan Lucic 4:24 into the second period — just Boston's seventh shot.
Marchand also took a penalty in the second, then came back to score. After Zdeno Chara kept the puck in the zone, Patrice Bergeron went to the faceoff circle to retrieve it, then passed it over to Marchand for an easy chip-in past Smith.
Smith started 20 games in the regular season and came on in relief of Dwayne Roloson twice in the series, shutting Boston out for the final 18:49 of Game 2 and then coming back in Game 4 with 2:02 left in the first period after Boston took a 3-0 lead. The Lightning scored five consecutive goals, and Smith shut out the Bruins for the final 42:02 to earn a 5-3 victory.
Smith's shutout streak lasted another 24:24 before Horton's goal tied it 1-1 in the second – a total of 85:15.
Notes
For the third time in the first five games of the series, a goal was scored in the first 80 seconds. … New England Patriots coach Bill Belichick was in the crowd, and he was voted fan of the game. Celtics point guard Rajon Rondo was also in attendance. … Ken Hodge, a star of Boston's 1970 and '72 Stanley Cup championships, also got a big cheer when he was shown on the scoreboard. … The Bruins almost scored on a dump-in in the final 3 minutes when the puck took a bad bounce and rolled right across the crease, inches from the goal line, as Smith was behind the net waiting to play it. … Boston went two entire power plays without getting off a shot. … The Bruins are 17-4 all-time when leading a series 3-2. … Tampa Bay faces elimination for the fourth time in this year's playoffs. The Lightning were down 3-1 to Pittsburgh in the opening round before rallying. … The Lightning fell to 8-2 in the playoffs when scoring first. … The Bruins' record low for shots in a playoff game is 12, against Toronto in 1951 and against Edmonton in the 1988 Stanley Cup finals.