Judy: Zellweger and Garland coexist symbiotically in the movie

"I'm only Judy Garland for one hour a night. The rest of the time, I'm part of a family." The rebuke is drawled during an interview on British television; an interview that is about to go south, tipping the already fragile Garland (Renée Zellweger) into the comforting oblivion of booze and pills once again. You get the sense that when she says it, she fully believes that being Judy is a part-time job or a costume she can shed at will. But what's particularly satisfying about this raw portrait of Garland at the end of her career, during a 1969 concert run in London, is how it taps into the fact that Judy never really has the luxury of being fully off stage. The Faustian pact with MGM studio boss Louis B Mayer, which turned a "fat-ankled, snag-toothed rube from Grand Rapids" into America's sweetheart, triggered a relationship with the spotlight that was every bit as addictive and abusive as her relationship with pills.

Adapted by Tom Edge from the stage play End of the Rainbow by Peter Quilter, and ably directed by Rupert Goold, this is a thorough workout for Zellweger's talents. It's not the kind of transformation in which the actor disappears into the character, like Marion Cotillard's Edith Piaf. Zellweger and Garland coexist symbiotically on the screen, in a kind of magic-eye illusion of a performance that flips back and forwards between the two. Zellweger is phenomenally good nonetheless. She captures the frayed edges of a voice that has taken a lifetime of punishment; the girlish ragdoll fling of arms and legs of a woman whose public never quite forgave her for growing up. And the humour: Garland's wisecracks are delivered with a trouper's comic timing.

But most of all, she nails the performance aspect. There's a stunning moment when Garland, hollowed out by exhaustion and paralysed by doubt, is forced on to the stage for the first of the London shows. She starts to sing, tentatively at first. But then the orchestra kicks in and her eyes snap open and suddenly a triumphant full wattage Judy Garland blast of charisma takes over. Backstage afterwards, she's crumpled and round-shouldered and smeared with tears. "What if I can't do it again?"

It's undoubtedly Zellweger's film. But this does mean that some of the other characters – Jessie Buckley's very clipped and British tour organiser in particular – can seem rather peripheral. And while the flashbacks to Judy's early years give a sense of the damage done to the studio's most valuable asset, I found them a little heavy-handed. Still, at its strongest, the film captivates; it grabs you by the heart and demands adoration. And in a film about Judy Garland, you would expect nothing less.