Colin Clark (Eddie Redmayne) is fresh out of university and wishes to work in film. His parents say he's crazy and tell him to stay home, but he is clinging to the idea that he met Laurence Olivier once, who told him that he would have a job with him when he graduated. So he goes to Olivier's production company headquarters in England and is told that Olivier isn't there. The man who is there says there are no available positions. So every day, he gets to the office bright and early to sit on the couch until Olivier shows up. Finally, Olivier (Kenneth Branagh) and his wife Vivien Leigh (Julia Ormond) arrive. Olivier is talking about what a struggle it's been to get Marilyn Monroe to come to England to film the movie "The Prince and the Showgirl." One of his assistants asks why he didn't just cast his wife in the role, and it is implied that Vivien is now too old and past her prime. Colin reintroduces himself to Olivier, who remembers him and gives him a job as third assistant director on the upcoming film shoot. Colin goes to stay at a cheap motel over a bar, where no one working there believes that he will be working with Marilyn Monroe.
Colin's first job is to find a suitable place for Marilyn Monroe and her new husband Arthur Miller to stay while they are in England. The paparazzi is going to be hounding them, so it needs to be somewhere private and unreachable. Colin finds a small, nice house and arranges for the owner to be paid to move out for the duration of the shoot. However, the paparazzi immediately finds out about the house. But Colin knew this would happen and had actually arranged for there to be two houses to throw the photographers off the scent. Olivier is very impressed with his work.
Colin tries to arrange for Marilyn Monroe's arrival to be kept secret, but the photographers swarm the airplane when it lands in England. Marilyn (Michelle Williams) has brought her husband, Arthur Miller (Dougray Scott), business partner Milton Greene (Dominic Cooper), and her acting coach Paula Strasberg (Zoe Wanamaker). She seems uncomfortable around the photographers but eventually handles herself well.
The first day on set, Colin sees a wardrobe assistant Lucy (Emma Watson) and seems to like her. Marilyn is hours late to the set, which frustrates Olivier. Once she does arrive for the table read, she insists that Paula sit with her, which goes against Olivier's directing style. But she gets her way, and when she has trouble with lines, Paula reads them for her. Marilyn whispers to Paula that she doesn't understand her character's motivation, and Paula announces this to the table. Olivier tries to explain firmly, but everyone else praises Marilyn for wanting to know her character inside and out; they all seem to cater to her, even the film's other star, the very respected Sybil Thorndike (Judi Dench).
One day, Marilyn forgets her script in her dressing room, and Colin is sent back for it. While he's there, he sees her massive collection of pill bottles. Marilyn comes to her dressing room but isn't angry at Colin for being there. Instead, she asks if everyone is horribly upset with her for being late and needing lots of takes for her scenes. Colin says no, and that everyone is just happy that she's there, himself especially. He calls her a great actress. She smiles and asks him to be her eyes and ears on the set. He agrees.
Marilyn begins arriving later and later, infuriating Olivier. She also defers to Paula when she has trouble with lines instead of asking her director, which is the common practice. But finally one day, she performs a scene where she's dancing, and everyone on set is captivated by her. This irritates Vivien, who, while Olivier is watching the footage back, breaks down and asks him if he prefers Marilyn to her. He assures her that he doesn't.
After some resistance, Lucy agrees to go out with Colin. They go dancing and end up back at her house kissing before her dad barges in. While it looks as though he's going to get upset, he instead shakes Colin's hand. Lucy looks smitten with Colin. However, shortly after, Marilyn doesn't show up for work at all. Instead, she calls the set and asks for Colin to come to the house where she is staying. He does, and they spend the day together, with Colin showing her the town, including the school he attended when he was younger. The photographers catch up with them, and Marilyn says, "Shall I be her? Marilyn Monroe?" showing that this is all a persona. She puts on a little dance for the photographers, and then runs away with Colin. She goes skinny dipping in a lake, which Colin follows her into. She kisses him before they are found by her security team.
One day on set, Milton Greene approaches Colin and tells him that he himself was in this situation before and that Marilyn likes to make men fall in love with her and then leave them brokenhearted. Later, Colin gets another phone call from Paula, telling him that Marilyn is asking for him. When he gets to the house, Marilyn has clearly been taking too much medication. She is sitting on the stairwell, crying over a book she found with Arthur's writings in them. They are sketches for a new play that clearly poke fun at Marilyn's persona, although Arthur denies any similarities. Arthur leaves to go back to the United States, and Colin puts Marilyn to bed, where she is almost incoherent. Colin lies next to her on the bed. In the middle of the night she wakes up in pain, clutching her stomach, shouting about how she can't lose the baby. A doctor comes and tends to her, but won't tell anyone whether or not she was actually pregnant. When Marilyn comes back to the set, Colin greets her, and she doesn't respond.
The film shoot finally ends and Marilyn is set to go back to the US. Lucy approaches Colin and asks whether Marilyn broke his heart. He says yes, and she says she's glad, because it needed breaking. While Colin is checking out of his motel, the bar goes silent, and the same people who didn't believe he was working with Marilyn see her walk into the bar. Outside, Marilyn says a proper goodbye to Colin, thanking him for his help and kissing him on the cheek. She drives off to the airport. Over the credits, there are snippets of what each character ended up doing with their lives. "The Prince and the Showgirl" ended up being Marilyn's platform to her most successful film, "Some Like It Hot" - the remarks on her leave out the matters surrounding her eventual death.
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