The Islanders were pretty far from their best selves on Saturday, but let a game they should have won fall through their fingers nonetheless.

Twice they had the lead against a Senators team placed 15th in the Eastern Conference, and even after blowing both, they got a lifeline in the form of a six-on-four goal in the final minute of regulation.

Still, they had to settle for a point after Brady Tkachuk sealed a hat trick with an overtime goal to give the Senators a 4-3 win at UBS Arena.

And thanks to the Red Wings simultaneously snapping their own losing streak in a win against the Sabres, the Islanders fell a point behind Detroit for the last playoff spot in the East, albeit with a game in hand.

“The bottom line, I mean, [we] just gave the game away,” Patrick Roy said. “That’s all we did.”

The offense that abandoned the Islanders in Los Angeles and Buffalo was a little more apparent back at UBS Arena, but not so much as to quell concern.

After going two straight games without scoring, the Islanders had their legs under them at the beginning of this one.

They forechecked and worked below the hash marks, spending considerable time in the offensive zone. The first line — in particular Mat Barzal — was noticeable on nearly every shift.

The net presence that Roy wanted, though, never quite showed itself.

And as the afternoon went on, the home side’s energy dwindled until they were the ones trying in vain to break out.

“I just think we got off to a good start and didn’t carry it over,” Matt Martin said. “There’s gonna be moments in a game, shifts in a game that don’t go well and then it’s up to the next group that hops over the boards to turn it back. … I think for too long, especially in the second [period], we just kept jumping over the boards with nobody really taking the bull by the horns and turning the game back in our favor.”

Even with a bad second period, the Islanders still went into the third holding a 2-1 lead after Kyle Palmieri’s go-ahead goal.

But instead of pressing their advantage, they went into a defensive shell and got the same results that strategy yielded so often earlier in the season.

It took just 6:35 for Ridley Greig to tie the game, finishing Tim Stutzle’s feed on an odd-man rush.

Then, just 15 seconds after Noah Dobson’s penalty for delay of game, Tkachuk wired in a one-timer from the slot to give Ottawa its first lead of the game with 7:46 to go.

The top-six that helped sparked the recent six-game winning streak was abandoned during the third period as well, with Palmieri and Barzal switching lines while Brock Nelson went to Bo Horvat’s right to accommodate Palmieri on his left.

That configuration started to produce some chances, and when Mathieu Joseph went off for hooking with 1:50 to go, it was on the power play to score for the first time since Sunday’s win over Anaheim.

That is exactly what it did, with Horvat’s one-timer from above the right circle tying the game with 37 seconds to go.

But instead of keeping their momentum in the extra period, the Islanders gave away a power play of their own as Horvat held Tim Stutzle.

Tkachuk then finished an odd-man rush after Adam Pelech tripped with the Senators entering the zone, ending the game in disappointment.

“It wasn’t enough,” Roy said of his team’s effort. “That’s for sure. Some of our guys made bad decisions or bad plays at key moments of the game. I felt we gave [them] those four goals.

“You’re looking at those four goals — when a team is not scoring as much as you wish, you gotta be, not perfect, but you gotta play well defensively or you gotta manage that puck and we didn’t. That’s what happened on those four goals. That’s it.”

The Islanders have done a lot of good work to get back into the playoff race. But it’ll all be for naught if they can’t figure things out now.

Their schedule this week is unforgiving, with the Rangers, Hurricanes, Red Wings and Jets on the slate — Thursday’s game in Detroit being perhaps their most important game left this season.

Things could look a whole lot different in either direction seven days from now.

“We just didn’t play our game tonight in my opinion and I think everybody can agree with that,” Horvat said. “Happy to get the point. But it shouldn’t have been that way.”