Mirren. McKellen. How could it possibly be that “The Good Liar” is the very first film in which the dame and the knight have co-starred? Bill Condon brings them together for this adaptation of Nicholas Searles’ novel, with a screenplay by Twin Cities playwright Jeffrey Hatcher. The twisty little tête-à-tête is a fine vehicle for the two charming British actors, but it’s potentially the politest, gentlest movie about a scammer ever.

This isn’t just a charming tale of two older people finding companionship in their golden years online. Although Roy (Ian McKellen) and Betty (Helen Mirren) admit to embellishing a thing or two on their first dates, Roy’s just a bit too ingratiating, rubbing Helen’s grandson Steven (Russell Tovey) the wrong way. His worry is justified: Roy’s an older, more dignified version of “Dirty John,” romancing elderly ladies with his sweet and nonthreatening demeanor, sussing out the size of their retirement accounts. With his trusty wire transfer keypad and posh “accountant,” Vincent (Jim Carter), Roy will drain anyone’s bank account.

But you don’t cast Helen Mirren in a role where she isn’t the more intelligent and cunning half of a pair. She plays a wounded widow with grace, but that’s not her true nature. So one spends the benignly bland first hour of the film waiting patiently for the other shoe to drop. And that’s an hour that includes a Russian butcher getting his hand tenderized and a man decapitated by a train. Somehow the beige-ness of it all just overwhelms everything, even the violence.

The beige-ness is kind of the point. It’s a misdirect, but it pervades every corner of the film like sleeping gas almost before it’s too late. When the other shoe does drop, and how (respect where it’s due), it’s a surprise that comes right out of left field. One can imagine that reading this as a novel would be filled with shocking suspense. But somehow, on screen, it lands with a “Huh?” instead of a “Gasp!”

“The Good Liar” takes its sweet time to pick up steam and pulls its punches in places where it could have been darker and more daring. Erring on the side of caution isn’t exactly the approach one should take when it comes to suspense thrillers. However, there’s more twist where that twist comes from, and Condon carefully lays out the pieces of the mystery. As it rounds the bend, doubling back on itself, the folds and bends and detail of the story revealed only in hindsight, “The Good Liar” finally steps into its full potential as a satisfying potboiler mystery.