It is always fun to watch the creases in McKellen’s face as the gears turn in Roy’s head, and the actor appears to get a kick out of switching on a dime between fragile Roy and menacing Roy. But during her offscreen time, Betty is — what? Could a retired Oxford professor really be d umb enough to fall for the idea that she and Roy, who have by this point started living together but are still platonic companions, open a joint account? Why would Mirren, no one’s notion of a damsel in distress, take a role that allows her to be little more than a helpless victim?

Asking such questions is the wrong way to approach this movie, which involves several layers of misdirection. Unfortunately, the director, Bill Condon, whose first film with McKellen was the much more interesting “Gods and Monsters” in 1998, is not an Alfred Hitchcock or a Brian De Palma — a filmmaker with a visual style seductive enough to offer distraction from the grinding plot mechanics, which are especially clunky here.

The sleight of hand “The Good Liar” tries to pull off might be easier to keep hidden on the page. As it progresses, the film reveals complications ( it plays particularly dubious tricks with the way it parcels out flashbacks to the 1940s ) and a motive that might as well have been picked out of a hat. The finale could be written with entirely different details, and almost no scene preceding it would have to change. The real good liar is whoever convinced Mirren and McKellen to class up such thin and arbitrary material.

The Good Liar

Rated R. Pervasive prevarication. Running time: 1 hour 49 minutes.