The film follows Dexter Mayhew and Emma Morley on the same day—15 July, St. Swithin‘s day—over 18 years. On 15 July 1988, after their graduation from the University of Edinburgh, Dexter and Emma spend a platonic night together and agree to “just be friends”.
One year later, Dexter helps Emma move to London to pursue a career as a writer. Finding little success by 1990, she ends up a waitress in a Mexican restaurant, where she meets Ian, an aspiring comedian. Meanwhile, Dexter travels the world, staying in touch with Emma. He visits her on 15 July 1991 and suggests they take a holiday. They go to France in 1992; despite their mutual attraction, Emma turns down Dexter’s advances. By 1993, Dexter is a successful television presenter with a raucous late-night show.
Dexter visits his parents on 15 July 1994 after his mother is diagnosed with terminal cancer. He arrives still under the influence from a night of heavy partying, infuriating his father. His mother is unimpressed with his wild lifestyle, and tells him he is not yet a nice person. That night, Emma goes on a date with Ian; despite their lack of chemistry, they begin a relationship.
By 1995, Emma is a school teacher and lives with Ian, but is increasingly irritated by his laziness. Dexter’s new show makes him “the most annoying man on telly”. Meeting Emma for dinner on 15 July 1996, Dexter gets high on cocaine, flirts with another woman, and insults Emma. She storms off and, deciding that they have outgrown each other, tells him that although she loves him, she no longer likes him.
Two years later, Dexter loses his television career, and by 1999, is in a serious relationship with Sylvie. Meanwhile, Emma has split up with Ian; he confronts her over their breakup and his jealousy of Dexter after reading her diary. Before leaving, he praises the stories in her diary and urges her to have them published. On 15 July 2000, Emma and Dexter attend the wedding of mutual friends. Dexter receives a job offer from Calum, his old roommate who has since become a multimillionaire. Emma tells Dexter she has received a book deal, and Dexter reveals that he and Sylvie are to be married, as he will soon be a father. Dexter and Emma rekindle their friendship.
By 2001, Dexter is a devoted father to his daughter Jasmine, unaware Sylvie is having an affair with Calum, and Emma’s book is published. Two years pass and Dexter, divorced, visits Emma, now a successful author living in Paris. Hopeful after a previous drunken tryst with Emma following his divorce, Dexter learns she has a new French boyfriend, and departs. Emma has second thoughts and chases after Dexter. They share a passionate kiss, and finally begin a relationship together.
Emma and Dexter are engaged by 2004 and eventually marry. Dexter opens a café in England, sharing custody of Jasmine, and he and Emma try, unsuccessfully, to have a child. On 15 July 2006, Emma is hit by a truck while riding her bicycle, and dies. Inconsolable, Dexter returns to his self-destructive habits. Over the years, he is comforted by Sylvie, by Jasmine, his widowed father, and even Ian, now happily married with children, who visits Dexter and tells him that Emma “lit up” around him, assuring Dexter that “she made you decent… and you made her so happy”.
On 15 July 2011, Dexter visits Arthur’s Seat in Scotland with Jasmine. The film flashes back to 1988: after their night together, Dexter declines Emma’s invitation to spend the day with her, but changes his mind. They climb Arthur’s Seat and at the top, Dexter suggests that instead of being casual friends, they “finish what they started” the previous night. They race back to the flat, but encounter Dexter’s parents on the street. Emma leaves, and Dexter tries to tell his parents she is just a friend, but chases after her to ask for her number. They kiss passionately, and promise to see each other again.