In their first film together, Helen Mirren and Ian McKellen match wits in a clever crime caper, set in London.

Widowed Betty McLeish (Mirren) is a retired Oxford professor with $3.6 million in assets and no idea how to invest her money. Tweedy, chivalrous Roy (McKellen) Courtnay is the professional con man she meets via an online service called Distinctive Dating.

Initially suspicious of one another, both use pseudonyms which are soon discarded as they realize they’re both looking for companionship and begin meeting regularly.

But when he’s not with Betty, scheming Roy and his partner-in-crime Vincent (Jim Carter) are planning to swindle a pair of businessmen (Mark Lewis, Stefan Kalipha) in a bogus offshore investment scheme.

One evening, when the restaurant at which they planned to meet is closed, genial Betty graciously invites Roy to her small suburban home where he meets Betty’s grown grandson Steven (Russell Tovey), who is writing a dissertation on Nazi architect Albert Speer.

As time passes, Roy introduces gullible Betty to Vincent, posing as his “accountant and financial planner,” who suggests that she and Roy combine their assets. They’re both delighted at what an easy “mark” Betty seems to be.

But then Betty insists that she and Roy go on a European vacation, stopping in Berlin, where Steven has been studying 1940s Germany. That’s where unexpected revelations and unpredictable complications arise.

Adapted by Jeffrey Hatcher (“Mr. Holmes”) from Nicholas Searle’s 2016 novel and directed by Bill Condon (“Gods and Monsters,” “Kinsey”), punctuated by Carter Burwell’s menacing score, it’s deceptively diabolical, leaving the audience to wonder who can be trusted and who can’t.

What makes this subtle cat-and-mouse game work are the combined talents of Helen Mirren and Ian McKellen, who play off each other with staggering finesse despite the film’s shaky credibility.