Unthinkable.

That the Boston Bruins and Bruce Cassidy are playing later into April than Claude Julien’s Montreal Canadiens.

That the Boston Bruins survived two late third-period power plays and two disallowed goals in double overtime to win the must-win Game 5 on the road. Carried on their backs by two college guys – Sean Kuraly (Miami of Ohio) and Charlie McAvoy (Boston University) – with a total of 15 career NHL games.

That Tuukka Rask played, arguably, his best game in a spoked-B jersey in Game 5, holding the Sens scoreless for the better part of three periods after his team fell behind, 2-0.

That the Bruins are still in a series in which they have led by a grand total of 2:33 in regulation.

Maybe Bruins’ history adds another page in the 2016-17 season Sunday afternoon in Game 6 – with David Krejci out. Ditto for Torey Krug and Brandon Carlo and Adam McQuaid.

“Down low, grind game,” Cassidy assessed pregame about the Sunday battle. “Less of a rush game. We have to win today.”

Indeed.

But not to be.

Here’s what we learned with no Bruins’ team in 22 playoffs coming back from a 3-1 hole to win a series. Make that 23 as their season ends in a 3-2 OT loss to the Ottawa Senators.

First-period hex broken despite undisciplined play

Go figure. The Bruins take three – yes, three – delay-of-game penalties in the first period, a first-ever in the NHL. The first on Sean Kuraly just 17 seconds in.

The Sens’ best opportunity in that opening 20 came when Erik Karlsson hit the post at 12:30 with Joe Morrow in the box for the second delay of game. And a game-saver by Rask with 16 ticks left when Mark Stone went in solo.

TD Garden a tentative mix of angst and anticipation all period until a delayed call sent Noel Acciari in alone at 17:25. No go for Boston’s first legit bid.

On the ensuing power play, however, Drew Stafford blasted a 15-footer from the right faceoff circle with Brad Marchand and McAvoy assisting at 18:13 for the 1-0 lead.

Finally. While Boston had outscored the Senators 8-4 in second periods, they were blanked, 3-0, in first periods into Sunday.

Sloppy second puts Ottawa on the board – twice

Another penalty to start the period at 2:49 on a McAvoy tripping call. Ottawa’s offensive juggernaut took little time in knotting the game when Bobby Ryan took a feed from Erik Karlsson to beat Rask at 3:26.

It added up to 12 points for the Ottawa duo, while Patrice Bergeron, David Backes, Marchand and David Pastrnak had 13 at the time.

At 8:32, Kyle Turris scored a pure NHL goal with a perfectly placed wrist shot from the slot – stick-side, top-corner – with no hope for a Rask denial.

Season on the line; Bergeron steps up early to create OT

Marchand sent a 50-footer from inside the blue line on Craig Anderson. It found crease – and Bergeron found the rebound, stuffing it home at 1:57; Colin Miller also assisting.

At 13:34, the B’s went on the man-advantage. No go for the go-ahead goal. They ended 12-3 shots on net

“Our power play only gets one,” Backes said. “So we lost the special teams battle and that’s how you lose games. Six one-goal games and three of four overtimes – it was pretty tight in there.”

MacArthur parked tight and right for Ottawa’s third OT win of series

David Pastrnak took two for holding at 5:54, and 36 seconds later, the season came to an abrupt end as Boston’s NHL-best penalty kill all season was tested – and bested.

“It’s our responsibility, again, to kill the penalties,” Cassidy said once again in the postseason. “Like I said, I don’t have a problem with the call. We just didn’t get it done on the penalty kill.”

It was the third OT win for the Sens – all on power-play goals.

“No story better than his,” Ottawa coach Guy Boucher said about his veteran, Clarke MacArthur, who missed the last two seasons. MacArthur was parked in front of Rask when Rask deflected a Bobby Ryan pass – onto MacArthur’s stick for the winning goal at 6:30.

“I kind of lost Bobby [Ryan] there,” Rask said about the winning goal. “He came behind the net I think and got in that dead area and you know I just got caught outside the post and I got the puck I caught off the pass but it went right to their guy.

“You play a seven-game series, the better team wins and I think they were the better team.”

Cassidy’s closing comments

“I think they just made a play or two in the games they won and we didn’t,” Cassidy said about the four losses.

He ended his last press conference of the season with three words when asked if he wants to return.

“Absolutely, 100 percent.”