Starring 93-year-old force of nature called June Squibb, she's out long past her bedtime, and she's not going back empty handed.
Thelma succeeds because it didn't realise it was meant to be a cosy, old folks feel-good movie. It thought it was a Mission Impossible action film. So why not be both?
Squibb finally hit the big time in 2014 with an award-winning performance in Nebraska, and since then has been in more than 50 films and TV shows. But this is her first time with her name above the title.
In fact, there really is a Thelma Post - the widow of Hollywood director Ted Post, who made some pretty good cop movies back in the 1960s and 1970s.
Thelma is also the grandmother of the director of this film, Josh Margolin. And she really was the victim of a heartless scam.
The real-life Thelma was stopped before she sent off thousands of dollars. But it got Margolin thinking - what if she hadn't been? What could she do?
Thereby hangs this tale - that, and a reminder that the star of the world's most successful action franchise Mission Impossible is no spring chicken himself.
What if Thelma decided to take on this scammer the same way Tom Cruise and the Missions Impossible gang would? The first thing she's going to need is a team, right?
But what do you do when your usual crew are dead or institutionalised?
Fortunately, Thelma thinks of the very man. I'll give you a clue. Who's the cat who won't drop out when there's danger all about?
It's private eye Shaft, of course, or rather actor Richard Roundtree who played him in the movie. He may no longer be a "sex machine to all the chicks", but there's one thing Thelma's friend Ben has got, and that's wheels - a mobility scooter for two.
So that's a reluctant sidekick and a slightly low-powered vehicle sorted out. Next, they need some firepower.
Thelma remembers her old friend Mona usually has a gun on top of her bedroom wardrobe. Time to go calling.
The next scene is pure Mission Impossible. While Ben distracts Mona, Thelma has to race up the stairs with Lalo Schifrin-type music pulsing underneath.
She climbs up on the bed, dislodges the pistol, gets off the bed and makes her getaway - no mean feat for a 93-year-old.
I should tell you, Squibb does most of her own stunts, apparently. Just like Tom Cruise.
Meanwhile Thelma's family belatedly notice she's gone missing - along with Ben and his scooter.
Stopping only for an extended family bicker, they take off after the world's oldest action heroes. The clock is clearly ticking.
But Thelma and Ben are way ahead of them as they hunt down the scammer's address. All they have to do now is wait for him to show himself.
And we're delighted to see that, like most action-movie villains, this one's an elderly Brit, Malcolm McDowell.
Our slightly shop-soiled Doctor Evil may whine that he's just trying to survive in these troublous times, but Thelma sees right through him.
Thelma is huge fun, and while it cheerfully ticks all the action genre boxes, it gives itself room for some notes on aging, on friendship and independence.
I lived with people all my life, says Thelma. I'm rather enjoying being on my own for a while.
For a film so clearly targeting the over-eighties, it's the most purely entertaining film I've seen in ages.
Squibb has waited long enough for her spotlight. And when she got it, she knew exactly what to do with it.
Borrow a granny and go and see Thelma. Neither of you will regret it.