Playing both sides of the law

Large yet puckish Robbie Coltrane -- forensic psychologist Eddie “Fitz” Fitzgerald of the British series “Cracker” and Rubeus Hagrid of “Harry Potter” fame -- is the main attraction of “The Planman,” a diverting if ultimately preposterous caper film that gets a domestic premiere Sunday night on BBC America. But he is in nearly all of it, so help yourself.

Coltrane plays Jack Lennox, a Glasgow lawyer who can no longer suffer gladly the fools who surround him. Like Leo Bloom innocently posing “a little academic accounting theory” in “The Producers,” Lennox’s stray remarks to a police detective he has just successfully defended on a corruption charge draw him into a second career as a criminal mastermind. (The detective, played by the excellent Vincent Regan, turns out to have been guilty.)

Using his superior brain and a volume on “systems thinking” -- this was called “getting the big picture” in my driver’s ed class -- he plans perfect robberies even while at work in court. (He is not such a mastermind, though, that he takes care to destroy his notes afterward.) These nobody-gets-hurt operations, gleefully portrayed, prove to be a (temporary) cure for his midlife crisis and failing marriage.

With Lennox dipping into crime as suits his mood (bad before, buoyant after), the film is basically amoral without having anything to say about amorality -- we’re on his side, whatever he does, without being called upon to reflect on our capacity to excuse him. All it takes to make one root for a crook, after all, is to pit him against an unlikable cop; Neil Dudgeon takes that assignment, as a self-important police bigwig sleeping with Lennox’s politician wife (Celia Imrie). And from the ‘60s-ish high-contrast graphics of the title sequence to naming a crime boss “Jam Jar,” it’s set up in a way that we won’t have to take it too seriously. When Lennox gathers the worst villains in Britain for the biggest score ever, their tutelage becomes, if anything, an occasion for a kind of feel-good humor.

As is typical of British productions, the locations work as an extra character and ground the proceedings in a reality the script doesn’t always supply. John Strickland, who has directed episodes of “Hustle,” “The Bill” and “Maigret” as well as “Prime Suspect 2,” keeps the action moving, even as it hurtles toward absurdity. And if it all falls apart at the end, well, by then it’s over.

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‘The Planman’

Where: BBC America

When: 8 p.m. Sunday

Rating: TV-PG (may be unsuitable for young children)