The Islanders staked themselves to a 2-1 lead after a strong first period last night, thinking about hopping a late-night flight to St. Louis with a five-game win streak tucked away.

They forgot about the remaining 40 minutes against the Bruins. The Isles turned pucks over and bumbled around their own end, leading to a 5-3 loss to Boston that snapped a four-game win streak and was the first regulation loss at Barclays Center this season.

Two turnovers directly led to Bruins' goals in the second period. And still within a goal as the third period marched on, another failed transition play led to a harmless pass going off both of Marek Zidlicky's skates and behind Jaroslav Halak, encapsulating the disjointed final two periods for the Islanders.

"We didn't do a good job complementing each other all night, forwards or D," coach Jack Capuano said. "We played Islander hockey in the first, but then I guess we thought it was going to be easy."

John Tavares had an assist on Josh Bailey's power-play goal that evened the game at 1-1 at 14:08 of the first, but Tavares and linemates Anders Lee and Ryan Strome had a poor night at even strength. Tavares got caught flat-footed in the slot as Brett Connolly zoomed past to beat Halak and open the scoring, then that line was on for another adventure in the defensive zone finished by Boston's David Pastrnak with a turnaround wrister past Halak with 1:44 left in the second to put the Bruins ahead to stay at 3-2.

"They raised their level in the second period and we didn't respond. It starts with me," Tavares said.

Halak pitched a 37-save shutout in Columbus on Tuesday by denying rebound attempts with the help of his teammates. Friday night, the Bruins had good chances from the slot that generated second and third attempts.

The Calvin de Haan-Travis Hamonic pair was on ice for the first three Bruins goals, with de Haan giving away the puck on Boston's second-period scores that came 2:03 apart. De Haan was pressured into throwing a clearing pass into the middle that turned into a golden chance; Hamonic blocked Ryan Spooner's shot but the puck deflected right to Joonas Kemppainen, who scored his first career goal to the tie game.

Pastrnak's goal came after a failed clear, a puck in the air that the Bruins forward had time to settle, turn and fire with de Haan unable to close.

"You could feel that swing happening in the second," Matt Martin said. "We just have to find a way to get out of it."

Even with that sloppy second, the killer came with 11:51 to play in the third. Ryan Spooner cut to the net, past Halak, and tried to throw a blind pass back toward the goal where only Zidlicky and Thomas Hickey were. The puck caromed off both of Zidlicky's skates and in.

David Krejci scored into an empty net and Hickey followed with one more Isles goal in the closing minutes for the final result.

"It was our mistakes that did it," Johnny Boychuk said.