Based on novelist Nicholas Searle’s best-selling 2016 debut, “The Good Liar” is a silly breeze of a movie starring two of Britain’s finest actors, each having a blast playing cat-and-mouse with the other.

Ian McKellen charms as Roy, a London con man who woos elderly women into signing their savings over to him. His target here is a seemingly unwitting widow he has met through an online dating service, Betty, played by the ever-delicious Helen Mirren.

The only question is when the rug will be pulled out – not from under Betty, but from under us.

On their first date, there are already hints that all is not as it seems, when Roy and Betty both confess to using fake names on their dating profiles. After the evening is over, Roy introduces us to the unsavory sideline he runs with his accomplice (Jim Carter): It’s a scheme involving an offshore investment opportunity and some Russians.

Meanwhile, Roy and Betty continue with their cute dates, including one in which the foreshadowing is a little too on-the-nose: After a screening of Tarantino’s historically revisionist World War II film “Inglourious Basterds” – “Liar” is set in 2009 – they commiserate about how young people all too often take stories at face value. Roy then puts his grift into action, insinuating himself in Betty’s life by feigning a knee injury so that he can move into her guest room in a secluded retirement community.

There are a couple of problems with this adaptation by screenwriter by Jeffrey Hatcher (“Mr. Holmes”), which largely sidelines Mirren as it focuses more and more on Roy’s other cons. But an even larger issue arises: Despite Betty’s lack of dialogue, no reasonably attentive filmgoer ever would imagine her – a retired Oxford professor who casually reels off her millions in assets – to be the easy mark that Roy expects her to be.

Betty’s skeptical grandson Steven (Russell Tovey) – a doctoral candidate – is not as delighted as his grandmother seems to be by her new houseguest, or his affinity for whimsically antiquated language. (This is the sort of puzzle of a movie in which words such as “tickety-boo” and “fond” play into the endgame). Steven, an expert on World War II history, immediately turns a suspicious eye on Roy, prying into aspects of Roy’s life, including how he got a certain scar from his military service.

Hatcher and director Bill Condon (“Beauty and the Beast”) ultimately stumble at the finish line. In an otherwise taut acting showcase, the film is punctuated by bits of jarring violence that threaten to throw the story off the rails. How Hatcher unknots all of the movie’s various twists is less than satisfying.

The story, which is ultimately about how we wish to portray ourselves, lands well enough, for the most part. But can anyone ever escape the past? The film doesn’t so much ask this as gesture to it, awkwardly. “The Good Liar” isn’t really about grand moral issues, anyway, beyond a simple fact: The echoes of a lie, however distant, never really fade.