One Day follows the story of Dexter Mayhew and Emma Morley, who meet and form a friendship as they graduate from university in 1988 and charts the progress of their lives by 'checking in' on their lives annually on the anniversary of the day they met.
This novel provides examples of:
- Amicably Divorced: Dex and Sylvie.
- Artistic License: Rex lost his job as a video game presenter in the movie because he is over 30 and considered too old for the job. Anyone remotely familiar with the gaming world would know that the average age of gamers is 35, and gaming Youtuber who are over 30 years old (the late The Cynical Brit and James Rolfe are just some examples) are just as popular as anyone else.
- Better as Friends: What Dex and Emma initially settle on.
- Betty and Veronica: Emma is the Betty to all of Dex's other love interests as Veronicas, while he is the Veronica to her Bettys (Ian especially).
- Book Ends: The first day Emma and Dex spent together.
- Disposable Fiancé: Ian. Their relationship is shown to be one more of convenience than passion.
- Mood Whiplash: From heartwarming to tearjerker in the space of one traffic accident.
- Shotgun Wedding: Implied between Dex and Sylvie once she got pregnant.
- Star-Crossed Lovers: Sort of. Their timing is off, and often when one is single the other is not, but also they both need to go through a significant amount of Character Development before being mature enough to handle their own relationship.
- Uptown Girl: Inverted. Dex comes from a wealthy, upper-class family and Emma from a more modest background.
- What the Hell, Hero?: Emma calls out Dex on his shallow, party-boy lifestyle. As does his mother.
- Your Cheating Heart: Sylvie cheats on Dex with his boss.
- Emma also has an affair with the married headmaster of the school where she teaches.