The Bruins had been playing well.

They could have created 5 points of breathing room between themselves and the third-place Maple Leafs in the Atlantic Division. They were coming off a season-high six goals against Philadelphia. They were playing the worst team in the Eastern Conference on Monday at TD Garden.

But if there’s anything to expect in this peaks-and-valleys season, it’s the unexpected.

The Bruins came out pan-flat in the first period, blew up in the second, and did nothing in the third. Appropriately, they were tagged with a 4-0 loss to the floundering Islanders.

The 17,565 fans at the Garden watched the Bruins fumble pucks, miss assignments, and play 60 minutes of disengaged and disinterested hockey. It would have been more entertaining for the fleeced full house to light their tickets on fire.

“It’s just not good enough,’’ said David Backes, who had one shot in 15:35 of ice time. “There’s no excuse-making going on. It’s just not good enough from top to bottom, left to right. We’re not going to win many games playing like that.

“If we want to play the way we did against St. Louis or Philly in here, we’re going to win a lot of games. We just can’t be Jekyll-and-Hyde and black-and-white on different days.

“That’s as frustrating as it gets. We showed we can do it. Now it’s imperative we do it every game, or else you see yourself starting to fall in the standings. That’s certainly not acceptable.’’

The Bruins’ 3-point lead over Toronto is deceiving. The Leafs have six games in hand. So do the Senators, who also trail the Bruins by 3 points. When the schedule eases in March and they see their chasers close or pass, the Bruins will feel the pain of lost opportunities like the one they had Monday.

The Bruins hadn’t submitted a dud in two weeks. The last stinker was a 3-0 no-show against New Jersey on Jan. 2. Since then, they went 3-2-1 while scoring 22 goals. They had seemingly straightened out their course. Pucks were going in. They were compiling more scoring chances than their opponents.

It is within that context that Monday’s no-show felt like even more of a boot below the belt to the Bruins. Both players and coach were mystified as to the reason for the decaf performance. But the product was clear: an egg big enough to cook an omelet.

“Flat tonight, obviously,’’ said coach Claude Julien. “Flat from the get-go.’’

Following a scoreless no-event first period, the Bruins allowed the winning goal at 13:18 of the second. It was the only evidence Julien needed of his players’ shortage of mental acumen.

During a battle in the Boston zone, Jason Chimera sticked the puck away from Adam McQuaid. Everything went poorly after that. Torey Krug tried to pursue Casey Cizikas behind the net. David ­Krejci wasn’t in position to seal off Cizikas’s pass. There wasn’t additional wing help in front.

The chain reaction of breakdowns led to enough space in front for a garbage truck to occupy. Nikolay Kulemin happily accepted the Bruins’ gift by darting to the slot, receiving Cizikas’s pass, and beating Tuukka Rask to give the Islanders a 1-0 lead.

“The goals you look at, we weren’t even in position that we’re normally in,’’ Julien said. “We were totally out of whack as far as even defending.

“When you give that first goal that much room in the middle of the ice, your D’s going in the wrong side, your weak-side forward’s out on the other side, and you open up the slot area, to me, that’s something I haven’t seen much of this year. It said a lot of our game tonight.’’

The gifts kept coming. Rask let Josh Bailey’s sharp-angle snapper slip between his lead pad and the near post at 15:00 of the second. Rask turned back Cizikas’s breakaway later in the second. But the Bruins overpursued the puck and gave Kulemin space in front once more. Before Patrice Bergeron could close on the backcheck, Cizikas connected with Kulemin again, this time at 18:38.

Jason Chimera added a shorthanded breakaway on Zane ­McIntyre at 12:00 of the third.

At the other end, Islanders goalie Thomas Greiss posted 32 saves. Not many were difficult. The Bruins’ second-line pairing of Backes and David Krejci combined for two forgettable shots on goal. David Pastrnak, parked in a 14-game goal-scoring drought, put just one puck on net. Bergeron and Brad Marchand put seven shots on goal, but Greiss turned them all back.

“It was really our whole team throughout the lineup that didn’t show up,’’ Bergeron said. “That’s obviously unacceptable.’’

Julien tried to spark his team in the third. He parked Rask on the bench. He shuffled his top two lines, which ended with Backes riding with Marchand and Bergeron, while Pastrnak was dropped to the second unit with Krejci and Ryan Spooner.

Nothing worked.

Fluto Shinzawa can be reached at fshinzawa@globe.com. Follow him on Twitter @GlobeFluto.