Two days earlier the Flyers had won 4-2 at Boston Garden to take a 3-1 lead in the best-of-7 series; Leach scored the game-tying goal in the second period. But at practice the next day, Leach got into an argument with the coaching staff and left the team's hotel in Valley Forge, Pennsylvania, to stay at his home 30 miles east in Cherry Hill, New Jersey.
Coach Fred Shero remained angry the next day and was inclined to keep Leach out of the lineup.
That's when captain Bobby Clarke stepped in. Clarke and Leach had a relationship that dated to their time as teammates with Flin Flon of the Western Canadian Junior Hockey League.
"It all boiled down to [Shero] didn't want me to play because of the things we had said and Clarkie wanted me to play," Leach said. "That was it. Clarkie said, 'Let him play, he'll play well.'
"I remember telling Clarkie, 'Don't worry about it. Just give me the puck. They're going in tonight.'"
Leach had scored at least one goal in his previous eight playoff games, so he had a reason to be confident.
He opened the scoring at 5:45 of the first period, beating Bruins goaltender Gilles Gilbert after taking a pass from linemate Bill Barber.
The Bruins tied the game 59 seconds into the second period, but Leach scored 17 seconds later to put the Flyers ahead 2-1. He scored twice more to give the Flyers a 4-1 lead entering the third, and scored again at 8:07 of the third period.
Leach became the fourth player to score five goals in a playoff game, joining Newsy Lalonde of the Montreal Canadiens against the Ottawa Senators in Game 3 of the 1919 NHL Championship, which was played prior to the Stanley Cup Final; Maurice Richard of the Canadiens against the Toronto Maple Leafs in Game 2 of the 1944 Semifinals; and Darryl Sittler of the Maple Leafs, who had done it two weeks earlier, on April 22, 1976, against the Flyers in Game 6 of the Quarterfinals. Since Leach, the only player to score five goals in a playoff game was Mario Lemieux of the Pittsburgh Penguins, who did it against the Flyers in Game 5 of the 1989 Patrick Division Final.
Leach's five-goal outburst helped the Flyers eliminate the Bruins and advance to the Stanley Cup Final, where they lost in four games to the Canadiens.
He finished the playoffs with a League-record 19 goals in 16 games (a total tied by Jari Kurri of the Edmonton Oilers in 18 games in 1985) and won the Conn Smythe Trophy as most valuable player in the playoffs. He's the only non-goalie from the losing team in the Final to win it.
Leach's playoff run was a carryover from his regular season, when he led the NHL and set a Flyers single-season record with 61 goals.
"That was an incredible year for me," Leach said. "I was the type of player that when I got my confidence scoring, I could pretty well score from anywhere. I used to tell my wife, 'I'm going to go to the game and I'll get a couple tonight.' "
