Rosie (Lily Collins) and Alex (Sam Claflin) have been best friends for as long as they can remember and plan to go to college in Boston together. Alex kisses Rosie on her 18th birthday; however, she is too drunk to remember it and Alex misconstrues her statement as wanting to stay Just Friends. Rosie gets pregnant by a classmate and decides to keep the baby without telling Alex, who goes on to Harvard without her. As the years go by, Alex and Rosie's devotion to each other — as well as their nebulous, never-addressed feelings — are tested by several complications.
- Bitter Wedding Speech: Rosie at Alex's wedding to Bethany.
- Childhood Friend Romance: Alex and Rosie have been best friends their entire lives and eventually develop feelings for each other; however, circumstances continue to keep them from acting on it.
- Disposable Fiancé: Thrice over. First there's Sally, Alex's girlfriend in medical school, who cheats on him. Then there's Greg, Rosie's high school fling and Katie's father, whom Rosie actually marries before he cheats on her. Finally, there's Alex's high school girlfriend Bethany, who marries Alex before they break it off and Alex can finally be with Rosie.
- Fourth Date Marriage: After finding out that Alex's girlfriend is three months pregnant, Rosie goes back to England, reconnects with Katie's father, and is married to him before the baby is born.
- Generation Xerox: Rosie's daughter Katie develops a very close friendship-bordering-on-romance with a boy named Toby; Rosie and Alex are reminded of themselves.
- New Old Flame: Rosie gets back together with her babydaddy Greg and Alex gets back together with his high school date Bethany at different points in the story.
- Poor Communication Kills: Part of the reason Alex and Rosie's relationship doesn't move forward is they never properly discuss the kiss they shared at her 18th birthday party.
- Teen Pregnancy: Rosie gets pregnant by Greg just as she's about to leave for college, and she has to put her schooling on hold. She later decides to keep the baby.