Katarina “Kate” (Emilia Clarke) is a young woman who works in an all-year-round Christmas shop with the boss who names herself Santa. Kate has been dealing with the heart surgery she had a year before that saved her life and her mother (Emma Thompson) is very overprotective. One day on Kate’s watch, the shop is robbed before she enters so Santa must make it seem like a break in to cover the fact that Kate left the store open. Kate is having rough family time as the family does not get along and they have no idea that her sister is a lesbian. Kate, one day, encounters a man who is observing outside and learns that his name is Tom.
After seeing Tom again a few days later, they decide to hang out and they become close after she has a failed audition. Kate learns that Tom works at a homeless shelter. Looking for Tom, who says he keeps his phone in a cupboard and often disappears for days at a time, she begins helping at the shelter in the hope of running into him, but finds that the staff have never met him, much to her confusion.
While celebrating Marta’s promotion, Kate spitefully outs Marta, a lesbian. She then runs into Tom, who takes her back to his apartment. Kate reveals that, a year earlier, she was seriously ill and had to have a heart transplant. Kate says she feels half dead and questions whether she has the talent to make it as a performer. After opening up to Tom, Kate tries to initiate sex, but he declines.
After spending the night with Tom, Kate decides to improve her life and help those around her and taking care of her body. Kate helps Santa with setting her up on a date with a man she has grown fond of—whom she names “Boy”—and tries to make amends with her family and friends, like apologizing for outing her sister Martha and helping people at the shelter. A few days later, wanting to see Tom, she tries to convince him to start a relationship with him but he declines which annoys her and she walks away angrily.
Finally, wanting to make amends with Tom, she returns to his apartment only to meet an estate agent who is holding viewings. After some initial confusion about who owns the flat, he reveals that the previous owner was killed in a bicycle accident last Christmas and the place has been vacant during the probate process. Finding his phone in the cupboard, Kate realizes that Tom was the organ donor whose heart she received. Heartbroken, Kate wonders around London and realizes that all their interactions were either hallucinations or spiritual apparitions. Going to a small garden, which was Tom’s favorite place, Kate encounters him again where he says his heart will always belong to her. Kate says a tearful goodbye to Tom as she disappears. The bench on which they sat during their first trip to the garden is revealed to be a memorial bench for Tom.
For Christmas, Kate organizes a show utilizing the talents of the people at the shelter and inviting all of her friends and family, including the newly coupled Santa and Danish man. Kate delicately performs a solo of the Wham! song, “Last Christmas”, intertwined with flashbacks of her so-called “encounters” with Tom until revelry ensues when the curtain rises and she is joined by the band of performers. Later, Kate and the family celebrate Christmas together joined by Alba, Marta's girlfriend, for the first time.
The Christmas celebration fades out and a bright light transitions to a summer scene where a visibly healthier Kate is seen writing in her journal in the garden to which Tom introduced her. Smiling and visibly happy, Kate looks up, as Tom always advised.
