By the time the first period ended, the Bruins had given Tampa three power-play chances and countless scoring opportunities, being outshot 14-4 over the first 20 minutes.
But while Gagne took advantage of his early chance - a turnover at the Tampa blue-line by David Krejci that led to a 2-on-1 with Steven Stamkos against Johnny Boychuk - the Lightning weren't able to take advantage of the numerous other opportunities that Boston allowed them in the first period.
When the lights came back up for the second act, they paid dearly.
It wasn't quite game two's five-goal period, but two tallies, one by Nathan Horton and another by Brad Marchand - both off of nifty passes from the strong-side wall by their linemates Milan Lucic and Patrice Bergeron, gave the Bruins what they ultimately wanted - a game five win, and a chance to close the series out in Tampa in game six.
The Bruins weren't great at getting pucks to the net - in the third period, it took them nearly 8:30 to get a shot on goal at all - but when they did, they got what they needed to out of them.
With around nine minutes to play in the game, Steve Downie finished a late hit on Johnny Boychuk, slamming his head into the boards and taking the Bruins' defenseman out of the game. Downie went for boarding, but the Bruins power play was again unable to produce any goals, despite finally using their big bodies down low.
The power play was the most productive effort the Bruins put forth in their five chances on the night, and helped to tip the ice back in their favor after Tampa had produced a very solid effort early in the third period.
Tim Thomas once again came up huge when he needed to be, stopping 33 of 34 shots for Boston, while Mike Smith - who hadn't been tested much in three-plus relief periods in games two and four, stopped 17 of 19 shots he faced for Tampa Bay.
When Tampa struggled to clear their own zone late in the game, Mike Smith wasn't able to get off the ice for the extra attacker.
Rich Peverley potted an empty-netter at 19:47 of the third to ice the game away.