Here was a game that looked straightforward, except to this year’s Islanders there is no such thing.

One of the traits that has defined these Islanders through 24 games is that they are the George Costanzas of the NHL, always doing the opposite.

So you get a successful road trip with two impressive wins, and then you get Tuesday — where the Islanders face-planted at home against the woeful Sharks by a 5-4 score on William Eklund’s winner at the end of overtime following a disastrous collapse in the last 12 minutes of regulation.

“We had the game under control with 8 ½ minutes left,” coach Lane Lambert said. “To lose that hockey game is a sin.”

He wasn’t wrong.

This capitulation materialized out of nowhere in a game the Islanders had dominated — not just in the first 40 minutes but in the opening part of the third period, when they extended their lead from 2-1 to 4-1.

It looked like they were finally asserting themselves with a lead instead of falling to pieces.

Turned out, they were just setting up for what might have been their most spectacular collapse of 10 blown leads in the third period this season.

Kevin Labanc started the trouble by making it 4-2 at the 11:55 mark, deflecting Nikita Okhotiuk’s shot from the left point.

Tomas Hertl cleaned up the garbage on Fabian Zetterlund’s rebound with 3:11 to go in the game, and that is where things started to feel all too familiar.

And with 1:30 to go, it was Hertl again at the back post burying Eklund’s feed and the Sharks jumping up and down in celebration and the Islanders stunned that they were living out a Groundhog Day nightmare of a hockey game that was now tied at four and going to overtime.

“It’s unacceptable,” Brock Nelson said. “That’s pretty much all that needs to be said.”

The Islanders were booed off the ice at the end of regulation, but it was the sound of disbelief echoing in UBS Arena after they exited the ice following an overtime where Noah Dobson and Bo Horvat both hit iron before Eklund scored the game-winner with five seconds left in the extra period.

“Guys have to make plays,” Lambert said, when asked how the Islanders can fix the problem that has underscored their entire season. “That’s it. You have the ability and we will continue to work on it. But in those moments, those plays have to be made.”

What makes this one especially painful is that until Labanc’s goal, things were going completely according to plan.

Even a Casey Cizikas penalty early in the third — the sort of thing that has traditionally been a precursor to collapse for the Islanders — was met only with a ferociously aggressive penalty kill.

Simon Holmstrom, out of gas as the power play expired, managed to get up the ice, recover a puck and feed Mike Reilly with pinpoint accuracy for Reilly’s first goal as an Islander and a 3-1 lead.

A pair of San Jose penalties minutes later set the Islanders up to make the rest of the game a formality. The Sharks managed to kill off 52 seconds of a five-on-three after Hertl tripped Nelson. But they could not stop the Islanders from scoring at five-on-four, with Ryan Pulock rocketing in a one-timer to make it 4-1 at 8:27.

That should have been all she wrote on a night when the top two lines were buzzing, when Julien Gauthier had scored for the second straight game and when the power play had netted a pair of goals.

The Islanders were doing all the things they had to do against the Sharks — holding the puck below the hashes, generating an effective forecheck, sustaining offensive-zone pressure and controlling the game.

Surely it was clear where this game was going, which meant that surely it was anything but.

The only common denominator is that doing all of those things for 60 minutes has been an ask this team has yet to consistently deliver on this season.

“It’s completely unacceptable,” Lambert said. “It’s happened too many times. It just has to stop. That’s it. Bottom line.”

Next time, maybe the Islanders should try ordering chicken salad on rye.