Linemate Alexei Kovalev had three assists as Montreal's top line combined for nine points and the Canadiens ended a six-game losing streak against Ottawa, including their three previous meetings earlier this season.
"Those guys have been so consistent," Canadiens defenseman Mike Komisarek said. "They're just flying out there. I'm sure glad I'm not playing against those three guys. They just seem to be finding each other all over the ice. They have great chemistry and they're getting chances and they're burying them, so they're a big reason why we've had so much success this season."
Mark Streit had a goal and an assist in the second, and Cristobal Huet made 26 saves as Montreal moved within one point of the Northeast Division-leading Senators, who also sit atop the Eastern Conference with 68 points.
"We all know nobody trusted us before the season," Plekanec said. "We were supposed to be in 13th or 12th place right now, and we are where we are and we know what we can do, but we're not going to get overconfident, that's for sure."
The Canadiens, who failed to make the playoffs last season for the fifth time in eight seasons, host Toronto on Thursday before another summit meeting in Ottawa two nights later.
"It's still 35 points to make the playoffs," Kovalev said. "That's the way I look at it, so we have to keep working."
Ottawa, which hadn't lost to Montreal in over a year, has lost eight of 11 since All-Star right wing Dany Heatley separated a shoulder in a 3-2 win over league-leading Detroit on Jan. 12.
"They're definitely playing well right now," Spezza said. "They're coming hard, we're not playing our best, but we feel when we get everybody back in the lineup and we get the guys we have in the lineup playing with the confidence we had at the beginning of the year that we can be the elite team we were last year and at the start of this year, and we still feel we've got a really good hockey club."
Martin Gerber stopped 22 shots, including a big stop on Kovalev's chance from the slot with just over four minutes remaining that kept the Senators' hopes of a comeback alive after they scored twice earlier in the third to draw within one.
Senators coach John Paddock used his timeout with 27 seconds remaining after Huet stopped Kelly, who shot from the right side on a cross-ice pass from Spezza.
"I felt we battled hard, especially in the third," Kelly said. "We were pretty persistent and it would have been nice to capitalize there. That was a great pass by Jason and you've got to give him credit. He made some nice saves."
After Kelly opened the scoring with his ninth goal 2:42 in, Kostitsyn drew Montreal even at 1 with a power-play goal at 12:11 when he got behind defenseman Andrej Meszaros to redirect Plekanec's pass across the goalmouth off the left post and into the net for his 16th goal.
Plekanec put Montreal ahead 2-1 with his 18th goal as he finished off a dazzling passing play with Kostitsyn and Kovalev.
"It looked fancy, maybe, but it wasn't that hard," Plekanec said. "It was just a simple play. We had two guys driving to the net and two passes and I had an empty net."
Kostitsyn drew his second straight assist on Streit's ninth goal 8:14 into the second as the Canadiens took a 3-1 lead right after Ottawa blew a chance to tie it on a three-on-two.
Senators center Mike Fisher couldn't handle Chris Neil's hard return pass to the right side and Montreal took advantage of Ottawa's near miss when Streit drove a slap shot from the left side past Gerber.
Kovalev got his third assist 10 minutes into the second on Plekanec's second goal. Streit also assisted as Plekanec banged a puck past Gerber at the right post for his 19th goal, Montreal's second power-play effort.
"Kovalev's playing some of the best hockey he's played," Spezza said. "He's controlling the puck and you've got to take away his time, and you've got to tip your hat. They scored some good goals tonight."
Spezza had a goal and an assist early in the third as Ottawa scored twice in 5:04 to draw within one with 12:14 remaining.
Spezza scored his 20th goal 2:42 in to make it 4-2 before he drew an assist as Vermette deflected Joe Corvo's point shot for a power-play goal at 7:46.
- Ottawa's third goal was originally credited to Corvo before a scoring change was made after the game.
- Senators captain Daniel Alfredsson missed his third game in a row because of a nagging hip injury. Alfredsson has played only three of Ottawa's last eight games, including a Senators-record seven-point game Jan. 24 in an 8-4 win in Tampa Bay.
- Canadiens D Roman Hamrlik missed his first game of the season because of the flu. Struggling RW Michael Ryder was a healthy scratch for a second straight game.
- Montreal hadn't beaten Ottawa since a 3-1 win on Jan. 29, 2007.