Following his graduation from university in 1956, aspiring filmmaker Colin Clark (Eddie Redmayne) travels to London to get a job on Laurence Olivier's (Kenneth Branagh) next production. Production manager Hugh Perceval (Michael Kitchen) tells Colin that there are no jobs available, but he decides to wait for Olivier, whom he once met at a party. Olivier and his wife, Vivien Leigh (Julia Ormond), eventually show up and Vivien encourages Olivier to give Colin a job on his upcoming film The Prince and the Showgirl, starring Marilyn Monroe (Michelle Williams). Colin's first task is to find a suitable place for Marilyn and her husband, Arthur Miller (Dougray Scott), to stay at while they are in England. The press find out about the house, but Colin reveals he secured a second house just in case, impressing Olivier and Marilyn's publicist, Arthur P. Jacobs (Toby Jones).
The paparazzi find out about Marilyn's arrival at Heathrow and they gather around the plane when it lands. Marilyn brings her husband, her business partner, Milton H. Greene (Dominic Cooper), and her acting coach Paula Strasberg (Zo  Wanamaker) with her. She initially appears to be uncomfortable around the many photographers, but relaxes at the press conference. Olivier becomes frustrated when Marilyn is late to the read-through. She insists Paula sits with her and when she has trouble with her lines, Paula reads them for her. The crew and the other actors, including Sybil Thorndike (Judi Dench), are in awe of Marilyn. Colin meets Lucy (Emma Watson), a wardrobe assistant to whom he is attracted, and they go on a date. Marilyn starts arriving later to the set and often forgets her lines, angering Olivier. However, Sybil praises Marilyn and defends her when Olivier tries to get her to apologise for holding the shoot up.
Marilyn struggles to understand her character and leaves the set when Olivier insults her. Colin asks the director to be more sympathetic towards Marilyn, before he goes to Parkside House to check on her. He hears an argument and finds a tearful Marilyn sitting on the stairs with Arthur's notebook, which contains the plot of a new play that appears to poke fun at her. Arthur later returns to the United States. Vivien comes to the set and watches some of Marilyn's scenes. She breaks down, saying Marilyn lights up the screen and if only Olivier could see himself when he watches her. Olivier tries unsuccessfully to reassure his wife. Marilyn does not show up to the set following Arthur's departure and she asks Colin to come to Parkside and they talk. The crew becomes captivated by Marilyn when she dances for a scene and Milton pulls Colin aside to tell him Marilyn breaks hearts and that she will break his too. Lucy also notices Colin's growing infatuation with Marilyn and breaks up with him.
Colin and Marilyn spend the day together and are given a tour of the library of Windsor Castle by Owen Morshead (Derek Jacobi). Colin also shows Marilyn around Eton College, and they go skinny dipping in the River Thames. Marilyn kisses Colin and they are found by Roger Smith (Philip Jackson), Marilyn's bodyguard. Colin is called to Parkside one night as Marilyn has locked herself in her room. Colin enters her room and Marilyn invites him to lie next to her on the bed. The following night, Marilyn wakes up in pain and claims she is having a miscarriage. A doctor tends to her and Marilyn tells Colin that Arthur is coming back and she wants to try and be a good wife to him, so she and Colin should forget everything that happened between them. She later returns to the set to complete the film. Olivier praises Marilyn, but reveals she has killed his desire to direct again. Lucy asks Colin if Marilyn broke his heart and he replies that she did, to which she replies that he needed it. Marilyn comes to a local pub, where Colin is staying, and thanks him for helping her. She kisses him goodbye and Roger drives her to the airport.
Output: Why is Marilyn in pain the night after she had locked herself in her room?