Washington killed off an early Boston power play and had an opportunity to jump out in front with a four-minute power play of its own after Lars Eller drew a double-minor for hi-sticking on Connor Clifton. But it was the Bruins who got the jump on the Caps, taking a 1-0 lead on Patrice Bergeron's shorthanded goal.
Boston's Brad Marchand blunted a John Carlson pass behind the Washington net, and the puck popped out to the front for Bergeron, who had enough time and space to put a backhander to the shelf at 12:02 of the first period.
Less than two minutes later, the Bruins doubled their lead. Boston got a stop in its own end and broke the puck out with numbers. A pass for Clifton failed to click, but he retrieved the puck from the right-wing corner and threw it to the net front where he hit David Krejci for an easy back-door tap-in at 13:56, putting the Bruins up 2-0.
In the waning seconds of the period, the Caps cut into the Boston lead. T.J. Oshie forced a turnover high in Washington ice and broke the puck out. He fed Nicklas Backstrom in neutral ice, and Backstrom skirted a Boston defender, skated in and took a shot from point blank range. Rask made the save for the Bruins, but Oshie chipped the rebound to the shelf to make it 2-1 with 9.9 seconds left in the frame.
Early in the second, Clifton continued his amateur dentistry career, administering a hi-stick to Conor Sheary and incurring a second double minor in as many periods. The Caps cashed in on both ends to take a 3-2 lead.
First, Oshie struck from the diamond off a fine feed from Evgeny Kuznetsov at 3:48 of the second. Oshie's second of the game made it 2-2. Sixty-six seconds later, Anthony Mantha slipped a shot through Rask - his fourth goal in as many games since joining the Caps in a trade with Detroit earlier in the week - from the right circle to give Washington a 3-2 lead at 4:54.
Boston rallied back quickly. Marchand tied it on a backhander from the slot at 6:33, right after David Pastrnak whiffed on a shot bid from virtually the same spot.
Late in the frame, the Caps killed off a penalty, only to yield a second Krejci goal at 16:02. The Boston pivot came into possession of the puck and waited out the Caps defense and Vitek Vanecek, putting a wrist shot to the shelf from the inside of the right circle to make it a 4-3 game.
Less than two minutes later, Boston got a 2-on-1 rush when John Carlson tried to keep a puck alive near the Boston line. Marchand carried into Washington ice and fed Pastrnak, who teed it up for the late-arriving Bergeron. The Boston center scored from the slot to make it a 5-3 game at 17:45 of the second.
Although the third period was scoreless, it was not uneventful. Washington had a pair of early power play chances - the first of which was a carryover from late in the second - but Rask shut down everything the Caps sent in his direction, including a pair of 1-on-1 chances in tight early in the third, the first on an Alex Ovechkin breakaway and the second on a Lars Eller follow-up of the Ovechkin rebound.
Midway through the period, tempers flared when a Tom Wilson hit on Sean Kuraly briefly sent the latter down the tunnel; he returned later in the game. Minutes later, officials used a television timeout to take a second look at a Garnet Hathaway hit on Boston's Jarred Tinordi, and they opted to mete out some justice in the form of a five-minute major and a game misconduct.
The five-minute penalty killing mission put a serious crimp in the Caps' comeback efforts, as did Rask's denial of Wilson's shorthanded breakaway bid. Boston kept the Caps at bay the rest of the way, and Marchand's late empty-netter accounted for the 6-3 final.
For Boston, the win was its fourth in a row since suffering the lopsided loss to the Capitals a week ago here.
"I think you can definitely see that confidence growing," says Bergeron, "I think that's what happens when you start getting that belief and knowing you can get back in games just like we showed today. Then you definitely play with a little bit of extra confidence and you can stay in the moment on every play."