Just the promise ofIan McKellenandHelen Mirrensharing the screen was enough to recommend “The Good Liar.” That the trailers suggested a diverting story about an old swindler out to fleece a well-off widow made the prospect even more attractive.

The reality is better than that.

Thisnew film from Bill Condon(“Kinsey,” “Gods and Monsters“) has detours and convolutions, wild turns and emotional shadings that can’t quite be anticipated. Those who don’t like “The Good Liar” may see it as too heavily plotted. Better to say that it’s a generous piece, with lots of story, and that it has the freedom to upend itself, shift gears and defy expectation.

Or to put it another way, walking in, you already know it’s going to be good. You don’t know that it’s going to bethismuch fun.

Over the opening credits, we meet Roy (McKellen) and Betty (Mirren) filling out forms for a dating service. Next, we see them at a restaurant having their first meeting. She is clear-eyed, white-haired and radiant, and he is slithery and charming. McKellen is best known to the public as the virtuous Gandalf from “Lord of the Rings,” but his curiously unreassuring smile, like a fox in the henhouse, has been used to good effect playingclassical villains such as Macbeth and Richard III.In “Good Liar,” Condon taps into McKellen’s simpering, glowering, serpentine side.

Based on the novel by Nicholas Searle, “The Good Liar” follows Roy through most of its running time. He is a con artist, who takes big risks for fairly low stakes — $25,000 here, $50,000 there. Betty, therefore, represents a potentially major score, and even then, she’s notthatrich. She’s just upper-middle-class and near the end of a lifetime of measured acquisition.

因为我们遵循罗伊,有一定的内部pull to root for him, as there is for any character that we follow. That’s why it’s an intelligent choice that Betty is not presented as wealthy, just a lot better off than Roy. If he succeeds in gaining her complete trust, and if he chooses to go through with his plans, she will be seriously damaged. We’re not talking about a mild reversal, but something that could send her onto the streets with a shopping bag. But then, if Maggie Smith and Bette Davis could play bag ladies, why not Helen Mirren?

So we watch with a certain degree of tension, the tension of liking both characters, but sensing that only one can end up happy. As Betty, whom we almost only see through Roy’s eyes, Mirren is trusting and warm, though we have to wonder a little bit about her intelligence, when Roy and his accountant start telling her about wonderful business opportunities in the Caribbean, and she actually seems interested.

The back-and-forth, the listening and reacting between Mirren and McKellen, as each of their characters gauges the other and as we mark the incremental shifts and exchanges of power, is pure pleasure.

For a long time, “The Good Liar” seems as if it will be no more than the story of a scam. But it flirts with being a love story and further flirts with the possibility that it might be a morality tale. Throughout, you may wonder, as I did, why is the movie following Roy? Why not follow both of them? By the time the movie is over, all is understood, and the answers are much more satisfying than anything you’d guess.

M“The Good Liar”:Drama. Starring Ian McKellen and Helen Mirren. Directed by Bill Condon.Theaters and Showtimes. (R. 109 minutes.)