The Golden Knights’ incredible inaugural season isn’t slowing down in the Stanley Cup final.

Tomas Nosek scored the tie breaking goal midway through the third period, and the expansion Golden Knights opened an improbable final with a thrilling 6-4 victory over the Washington Capitals on Monday night.

Marc-Andre Fleury made 24 saves in an occasionally shaky performance, but the three-time Stanley Cup winner’s new teammates carried the goalie who has so often carried them with a relentless outburst of offense.

The Capitals, the Eastern Conference champions, hadn’t given up this many goals in any of the 29 games they had played since March 18, but they hadn’t seen anything like this charmed run by the upstart Knights.

“We put fun ahead of everything, and you can tell,” said Ryan Reaves, who scored the Knights’ tying goal in the third period. “Guys are having fun and they’re smiling.”

With its sellout crowd of hometown fans at deafening volume all night, Vegas put its usual speed and relentlessness on full display while overcoming a third-period deficit to win the opener of a matchup between two franchises seeking their first Stanley Cup titles.

The Game 1 winner has won the last six Cups and 61 of 78 overall.

Braden Holtby stopped 28 shots for the Capitals, whose first Stanley Cup final game in 20 years was a defensive nightmare. Washington still had chances to win, but never slowed the Knights.

Tom Wilson got credit for the goal that put the Caps up 4-3 early in the third period when Fleury back-heeled a loose puck into his own net, but Reaves evened it 91 seconds later for Vegas.

Nosek then put the Knights ahead after Shea Theodore kept the puck in Washington’s zone, sidestepped a defender and fired a beautiful cross-ice pass Nosek, who buried a one-timer for his second goal of the playoffs.

Colin Miller, William Karlsson and Reilly Smith scored early goals before Nosek added an empty-netter for the Knights, who are three wins away from one of the most improbable championships in recent North American team sports history. Just 342 days after the Knights selected the backbone of their first roster in the expansion draft, Vegas had another party on the Strip with its remarkable collection of castoffs.

Brett Connolly, Nicklas Backstrom and John Carlson scored for Washington, but its biggest stars didn’t match the Knights’ outburst.

Captain Alex Ovechkin, who collected a career-high 22 points in the first three rounds, had one assist in his first Stanley Cup final game. Evgeny Kuznetsov, who scored 24 points in the first three rounds, also had just one assist.

Vegas is aiming to be the first franchise in a major North American professional league to win a title in its first season since the 1950 Cleveland Browns, who entered the N.F.L. as a fully formed team after four seasons in the A. A. F. C.