Hamrlik took Plekanec's centering flip pass from the right side on a 2-on-1 and put a shot into the open left side.
"He made a nice Czech sausage, that's what we call it," Hamrlik said. "Sometimes in practice when you do a nice flip we call it a Czech sausage. He made a great pass. It was perfect, I couldn't miss it."
Jeff Tambellini drew New York even for the second time 15:24 into the third with his second power-play goal of the game.
Jaroslav Halak made 29 saves in his fourth straight start for the Canadiens (6-5-0), who finished a 4-2 homestand.
Glen Metropolit, who was ill earlier in the day and didn't practice, assisted on both Montreal goals.
"He was at home and we asked him to rest," Canadiens coach Jacques Martin said. "We touched base with him at 4 (p.m.) and he showed up at the rink and felt that he was better and he just thought that he'd try in the warmup, see if he had enough energy, and obviously he was all right to play."
Mike Cammalleri, who scored his third career hat trick Saturday, extended his points streak to five games with an assist on Moen's third goal of the season which restored Montreal's one-goal lead.
Moen began the play that led to his goal behind the Islanders' net when he passed the puck along to Cammalleri, who darted towards the left corner before turning back to fire a centering pass to Moen, who redirected the puck past Martin Biron.
"It was just a good shift," Moen said. "We were cycling in the corner and I saw [Cammalleri] get the puck with a little bit of room so I just parked myself in front and got a stick on it."
Spacek scored 13:34 into the first as the Canadiens beat the Islanders at the Bell Centre for the second time in five days, including a 5-1 win on Thursday.
"It's not too strange," Cammalleri said. "It helps because your familiar in a way with the team. Your pregame meetings don't have to be quite as long so the coaches get a little bit of a break that way."
Biron stopped 28 shots for New York (1-4-5). Biron made 38 saves in Thursday's loss, which began the Islanders' current 0-1-2 skid.
"We're playing some great games," Biron said. "We've played one game that was really, really bad out of 10, and that was the game against Montreal here last week. We've gone to overtime six times so one bounce one way or the other and it could be a different thing and that's the positive we've got to take out of it."
John Tavares, the top overall pick in the June draft at the Bell Centre, was held without a point for a fourth straight game. He recorded four goals and three assists in New York's first six games.
Islanders captain Doug Weight did not make the trip. Weight, who is suffering flu-like symptoms, missed his third straight game.
Canadiens forward Maxim Lapierre's face had a close call with Brendan Witt's skate blade when he tripped the New York defenseman along the boards behind the Islanders' net 6:05 in. Lapierre left the game briefly to go to the dressing room before returning.
Tambellini tied it at 1 with 1:42 left in the second when he cut hard to the net from the right side and beat Halak between the pads with a shot along the ice for his second goal.
"Jeff's a guy who's got a boatload of talent," Islanders defenseman Andy Sutton said. "He needs to get some confidence at this level and he can really score."
- Weight, who began the season with a groin injury, has been limited to two assists in five games.
- G Carey Price sat for a fourth straight game after starting six of Montreal's first seven games. Price, whose next appearance will be his 100th career game, is 2-4 with a 3.36 goals-against-average and a .886 save percentage.