Two legendary British actors have the time of their lives in a snack of a movie that's rather satisfying in its old-fashioned way.

On their first night out together after meeting on a dating site for widowed septuagenarians (OK Boomer?), Roy Courtnay (Ian McKellen) and Betty McLeish (Helen Mirren) take in a screening of “Inglourious Basterds” at a London cinema. It’s the summer of 2009, and neither of these characters knows what happens at the end of Quentin Tarantino’s revisionist World War II extravaganza. Leaving the theater, Roy scoffs at what they’ve just watched: “Young people will think that’s what actually happened.” Betty isn’t so sure — she argues that the modern world has done more to clarify history than it has to obscure it.

More than just a crafty bit of foreshadowing (the full mirth of which isn’t clear until almost two hours later), this early scene appears to set up a pleasantly middle-brow thriller about our power to alter the past. If Bill Condon’s “The Good Liar” never manages to follow through on that promise — despite gesturing broadly in that general direction from time to time — this old-fashioned snack of a movie is still rather satisfying in its way. Pleasant and preposterous in almost precisely equal measure, the film never offers anything less than two all-time British actors having the time of their lives, which makes it hard to get frustrated that it seldom offers anything more. As far as Condon/McKellen collaborations (this is their fourth), it’s manna from heaven compared to “Beauty and the Beast.”

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When Roy first meets Betty over a spot of lunch, the two lonely hearts try to cut through the bullshit as fast as they can. They’ve only got so much time left on the clock, and so there’s no use beating around the bush. Roy — a suspiciously affable old bloke who’s prone to heavy breathing and warbling jowls — bemoans that online dating is “a system for matching the delusional with the hopeless.” Betty — a bottle of distant starlight who seems a mite too gullible and happy-go-lucky for a retired Oxford professor — doesn’t ask which side of that coin she’s meant to represent. They both admit that they lied about everything on their dating profiles, and resolve to be honest with each other. But the closer one gets, the more they lose perspective. And that cuts both ways, across delusional and hopeless alike.

It’s obvious from the outset that Roy isn’t the harmless fop he pretends to be. On the contrary, we learn almost immediately that he still works as a low-rent grifter who spends his free time hatching elaborate wire fraud schemes with his right-hand man (“Downton Abbey” star Jim Carter) and softening us up with cute slang like “tickety-boo.” And while it’s hard to fathom what an aging grifter might want from a sweet lower-upper class grandma, it’s clear that Roy’s burgeoning companionship with Betty isn’t on the up and up — even before he fakes a knee injury that leads him to move in with her.

Betty is a bit suspicious, herself. For one thing, her house feels as inert and unlived in as an Ikea showroom. For another, she tells Roy her net worth (a cool €2.8 million) as if she’s never thought about it before. Last but not least — and even in her vulnerable state — she’s just too damn smart to believe Roy when he tells her that combining their assets might reduce her financial exposure and lead to regular “windfalls” of new cash. And yet, to the horror of her peevish grandson Steven (Russell Tovey), Betty falls for it head-first. They just have to kill some time while Roy moves everything into its right place. Perhaps a scenic trip to sunny Berlin will be in order? Surely there’s not a twisty, Holocaust-adjacent backstory waiting in the wings of desire over there.

Screenwriter Jeffrey Hatcher doesn’t always find the most elegant way of untangling this ridiculous web of deceit, nor does he arrive at a perfect workaround for the lengthy time period covered by Nicholas Searle’s novel of the same name. Whereas the book was interspersed with flashbacks to Roy’s time in the war, Hatcher’s script collects them into two large chunks. And while the book took a similarly piecemeal approach to Roy’s criminal history, Hatcher’s script folds it into a present-tense storyline that runs parallel to the rest of the action. It’s amazing how many laws a 79-year-old crook can break in a single afternoon! After a shockingly violent sequence in the bowels of the London Tube, it becomes impossible to tell if Roy is an artful codger or Professor Moriarty.

Then again, that’s the idea; the central question of “The Good Liar” isn’t whether Roy is decent or dishonest, but rather if he’s too crooked to ever go straight. And the efficient shortcuts that Hatcher takes allow McKellen to toy with our sympathies like a cat with a bird carcass. Betty might be quick to absolve Roy by saying that his secrets are “between you, God, the Devil, and the dead,” but McKellen wrings a ton of fun from that tug-of-war. Watching his cheeks swell and crumple with every heartfelt twinge and half-truth is worth the price of admission unto itself, and Condon’s clean direction makes sure that we never miss a single moment of empty bluster. Tobias A. Schliessler’s cinematography leads McKellen into the darkness, as the film gets seduced by shadows as it careens towards the finish (the final scenes are just evocative enough to make you wish that Condon had tipped the whole thing into full-blown chiaroscuro noir).

And yet, it’s Mirren who’s ultimately asked to carry this movie across the finish line, and she does so with oodles of her signature elan. It’s a rare actor who can split the difference between an airport thriller and a historical reckoning — who’s able to conflate the silly with the serious in a way that completely erases the difference — but Mirren is more than up to the challenge. The final stretches of Condon’s film are so ludicrous that you almost feel swindled for caring about the movie until that point, but Mirren grounds one plot twist after another with the gravity of her conviction (and a little help from Carter Burwell’s lilting, uneasy, “Mr. Holmes”-esque score). “The Good Liar” may not have much to say about redemption, entrapment, or the fibs that can hold a friendship together, but the past is only so important to a wicked little thriller that delights in the moment at hand.

Grade: B-

Warner Bros. will release “The Good Liar” in theaters on November 15.

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