Every great movie star has a thing. Brad Pitt likes to eat. Woody Allen stammers. Julia Roberts smiles that toothy smile.

Tom Hardy mumbles through incredibly thick accents.

This is an unusual signature in an art form that tends to rely pretty heavily on the audience’s ability to understand the dialogue spoken by the actors. But in film after film — Bronson, The Dark Knight Rises, Child 44, Mad Max: Fury Road — Hardy gives viewers’ ears a serious workout. That’s doubly true of Legend, since he plays two different characters; twin gangsters Ronnie and Reggie Kray, who came to control much of the organized crime in London in the 1960s (while conversing in heavy Cockney twang).

Though Legend is the Krays’ story, it’s told from the perspective of Frances Shea (Emily Browning), Reggie’s longtime girlfriend. The blend of romance and true crime immediately makes Legend feel like the British version of Goodfellas (“Goodblokes”?), and that’s before writer/director Brian Helgeland tries to recreate (and then outdo) the famous Goodfellas Copacabana scene by following Reggie as he takes Frances on a date to his nightclub. In a single take, the camera observes the couple as they greet customers and find a table, and then accompanies Reggie as he excuses himself to attend to some gangster business. The shot might be derivative, but it’s still impressive in the way it suggests the intermingling of violence, power, and celebrity in the Krays’ universe, and conveys the lack of distance between Reggie’s personal and professional lives.

Reggie is, relatively speaking, the “good” brother, and Hardy plays him with an air of natural swagger and charisma (and his perfectly tailored suits are, objectively speaking, f—ing incredible). But while Reggie can pass for a respectable businessman, Ronnie is a loose cannon. A paranoid schizophrenic with a perpetual (and comical) pout, he refuses to take his medication, which makes him prone to bursts of uncontrollable violence and generally bizarre behavior, like parading a donkey through the Krays’ casino. Reggie keeps nudging the brothers towards respectability; Ronnie’s madness repeatedly threatens to destroy them both.

Hardy’s twin magic is outstanding. He makes Ronnie and Reggie convincingly distinct, and his shared scenes opposite himself (including a ferocious fist fight) are the most impressive of their kind since The Social Network. But the key word in the last paragraph was “repeatedly.” Despite its technical wizardry, Legend quickly settles into a monotonous and predictable groove: Reggie tries to make good, Ronnie does something bad, Reggie bails him at, his relationship with Frances suffers, lather, rinse, etc. Reggie is too loyal to turn in his brother, so the cycle begins again and again. If insanity is doing the same thing over and over and expecting a different result, then Legend might be crazier than Ronnie ever was.

Even at 130 minutes, Legend barely digs into the brothers or their enterprise. The period details look beautiful (as does the photography by Dick Pope), but they’re lacking in specificity. It’s impossible to say how much time passes during the film, or even to understand the exact nature of the Krays’ criminal enterprise or the extent of their reach into the British underworld. You’ll have to do some reading on the internet to learn how the Krays got their start, or to discover that one of the character’s fates was a lot more complicated than the movie makes it seem. It’s not a good sign when your biopic is less authoritative than your subject’s Wikipedia page.

What lingers after Legend’s over are a handful of scenes; most involving Ronnie’s insanity and his charming openness about his homosexuality, which he cheerfully shares with everyone, including an important player in the Meyer Lansky’s crime syndicate (Chazz Palminteri). As usual, Hardy’s a little tough to understand at times, but he’s rare actor who can make uncommunicativeness compelling. The YouTube video that collects of all of his best moments from this movie is going to be epic. Just hope it comes with subtitles.