DENVER — The Giants left the Mile High City down in the dumps after a collapse for the ages.

Perfectly positioned for a season-changing victory, the Giants went from pitching a 45-minute shutout Sunday to allowing four fourth-quarter touchdowns, blowing an 18-point lead in the final six minutes and wasting their own improbable comeback in a 33-32 walk-off loss to the Broncos.

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“There’s no way in hell we are supposed to lose that game,” Brian Burns said after he had wiped the tears from his eyes. “We gave that to them.”

Whispers bounced off the walls in a locker room silenced by turning a 99.8 percent win probability (per ESPN analytics) into the first loss by the past 1,603 NFL teams that held an 18-plus point lead with six minutes to go.

What was the level of shock after Wil Lutz’s game-winning 39-yard field goal?

“An emotional roller coaster like that is brutal,” tight end Daniel Bellinger said. “We’re going to be thinking about it for a while.”

So, how did it happen that Jaxson Dart’s 283-yard passing, four-touchdown (three passing, one rushing) performance, Theo Johnson’s unlikely touchdown catch and a defensive performance littered with big hits and forced punts became marred by allowing 33 fourth-quarter points and a walk-off field goal?

The collapse started innocently enough and ended in a horror show.

“We’ve been in the situation where we could’ve had a team suffocated and some way we give them life,” defensive tackle Rakeem Nunez-Roches said. “There goes the change that we need. And we never recover from that.”

The Giants led 19-0 at the end of the third quarter and 26-8 with less than six minutes remaining after the teams traded circus touchdowns off tipped passes — first by Troy Franklin and then by Johnson.

But the Giants (2-5) had given back their entire lead when Bo Nix scrambled for an 18-yard touchdown and a 30-26 lead with 1:51 remaining.

“We didn’t finish in the fourth quarter,” Dexter Lawrence said. “We’ve got to win that one.”

The lone silver lining could be Dart and how he responded to throwing a brain-fart third-down interception that gave the Broncos a short field in the middle of the frenzied comeback.

“I’ve got to be way better. That s- -t is unacceptable,” Dart said. “We were in full control of that game.”

Trailing for the first time all game, Dart completed a 19-yard pass on fourth-and-19 that included a 15-yard tack-on penalty for roughing the passer. He finished the drive with a 1-yard quarterback sneak for a touchdown that should’ve saved the Giants from devastation if the defense could hold for 33 seconds.

But Jude McAtamney missed his second PAT, opening the door at 32-30 for what every tortured Giants fan knew was coming next.

“That’s on me,” McAtamney said. “The snap and hold were good. It was my lack of execution on the kicks. I’m not going to shy away from that.”

A four-play, 56-yard drive that included big catches allowed by Dru Phillips and Deonte Banks — the benched former first-round pick only in the game because of Paulson Adebo’s injury — ended in the second walk-off field goal to beat the Giants this season.

Head coach Brian Daboll ripped off his hat, spiked his headset and yelled in the direction of defensive coordinator Shane Bowen as the game neared its end. But Daboll — who butted heads with his last defensive coordinator, Wink Martindale, for being too aggressive at times — claimed it was because “we lost the game” and not because of calling soft coverage.

“That’s a tough way to lose,” Daboll said. “A lot of plays that could’ve changed the outcome of that game. Not a lot of talking needs to be done when you lose a game like.”

The fourth quarter — Nix’s two touchdown passes, two two-point conversion passes and two touchdown runs, in particular — made no sense on top of the first three quarters.

Not if you saw the Giants offensive line push around one of the NFL’s best defenses. Not if you saw the defense rattle bones in a way that would make their 1980s predecessors proud. Not if you saw Dart and Cam Skattebo (13-yard touchdown catch) continue their rookie magic.

Nothing felt impossible if the Giants were going to follow up wins against the Chargers and Eagles by beating the Broncos on the road and start thinking of big trade-deadline acquisitions to fuel a playoff push.

“This one hurts,” Lawrence said. “This one hurts. This one hurts.”

The Giants scored touchdowns of 44, 41, 31 and 13 yards, and Dart found three different targets for touchdowns to make it like he wasn’t handcuffed by an injury-plagued receiver corps.

Dart’s off-script playmaking, defensive penalties by the Broncos and a shutout defense were three of the main ingredients in building a 13-0 halftime lead.

The Broncos punted on each of their first four possessions and got stopped at the goal line on their fifth as the Giants acted like enforcers.

When Tyrone Tracy Jr. broke away for a 31-yard touchdown run late in the third quarter, — with so much empty space around him that he was searching for a penalty flag — the game seemed well in hand.

In reality, it was the prologue to a nightmare.

“I put that all on us. We have to play better,” Burns said, deflecting blame from Daboll’s renewed hot seat. “Maybe there might have been a sense of getting a little comfortable — we had a nice-sized lead and we have to act like we’ve been there before. You can’t let up because you see what can happen. That’s a tough loss for me…. I don’t like that s–t.”