Montreal won for the third time in four games and made things miserable for both the goaltenders used by the Senators.
The Canadiens had only a 2-1 lead heading into the second period but Brian Gionta, Max Pacioretty and Plekanec all scored in the second. Plekanec also had a first-period goal and ran his total for the year to 16.
Carey Price made 33 saves to get the win.
Mike Brodeur opened in goal for the Senators and gave up four scores on 15 shots before being removed in the second period. Brian Elliott stopped 17-of-20 shots for the remainder of the game.
Ottawa has lost five in a row and 11 of its last 12.