When Marilyn Monroe walked into a room, men and women froze in her orbit. Athletes and intellectuals fell before her — presidents, too.

How does an actress portraying this Hollywood icon convey such magnetism? In “My Week With Marilyn,” Michelle Williams disappears so effortlessly into Monroe’s translucent skin that the camera lens seems to fog up with desire. She’s that good.

The year is 1956 and Monroe, at the height of her pin-up popularity, is making a movie in London with master actor Laurence Olivier. Behind the scenes she is a complete mess — doped up, paranoid and suffocating in insecurities.

Monroe finds solace in one of Olivier’s young assistants, Colin Clark (the film is based on his memoir). It’s an unlikely pairing as Monroe is in the midst of her third marriage, this time to Arthur Miller. Told through the kid’s eyes, “My Week With Marilyn” is burdened with an unnecessary narration. While Colin’s expository musings move the story along, they also tend to snap us out of the euphoria produced by Williams’ performance.

The actress’s gritty roles have won her Oscar nominations (for “Brokeback Mountain” and “Blue Valentine”). Her Monroe is something else entirely. Williams stalks through the film, both as a sexual nymph bent on conquest and, at times, a childlike victim, injured and afraid.

As Colin, the British actor Eddie Redmayne is sufficient, but inevitably overshadowed by the British acting royalty who portray their 1950s equivalents. Kenneth Branagh is hilarious as Olivier while Dame Judi Dench devours her scenes in a brief turn as aging actress Dame Sybil Thorndike.

Colin’s fleeting moments with the most famous woman of the century provide a further glimpse into Monroe’s complex identity. And Williams’ performance is the key. With it, she unlocks the beauty, the raw talent and the destructiveness that was, in the end, Marilyn’s undoing.