Rosie Dunne and Alex Stewart have shared a deep-rooted friendship since childhood. The film begins with Rosie feeling despondent while seated at a table, surrounded by wedding guests celebrating. Flashing back to the past, we see Rosie and Alex delighting in the festivities of Rosie's 18th birthday at a bar. In a moment of spontaneity, they share a passionate kiss, but chaos ensues when Rosie falls off her stool, knocking herself out. The next day, unaware of their kiss's significance, Rosie expresses regret about that night, leading Alex to mistakenly believe she's rejecting him.
Their bond faces further scrutiny when they attend a beach bonfire and discuss the impending school dance. Despite turning down an invitation from Greg, the school’s most attractive boy, Rosie feels torn as Alex finds himself drawn to a new girl, Bethany. Rosie, hoping to support her best friend, encourages him to pursue Bethany, even as Alex reveals his newfound sexual experience after his night with her. During the dance, they awkwardly navigate their feelings, sharing stolen glances from across the ballroom. However, a mishap arises when a condom becomes stuck during Rosie's encounter with Greg, prompting a call for help to Alex, who abandons his own plans to assist her.
As the story unfolds, Alex earns a Harvard scholarship and Rosie gets accepted into a hotel management program at Boston University, and they make plans to venture to the United States together for college. Everything changes for Rosie, however, when a pharmacy visit reveals she's pregnant with Greg's child. Torn between confiding in Alex and fearing he’ll sacrifice his opportunities for her, Rosie opts to carry the baby to term and give her up for adoption. This decision parts them, and they share a heartfelt goodbye at the airport.
Back in England, Rosie welcomes a daughter named Katie into the world, ultimately choosing to keep her and embrace motherhood. Time passes, and as Katie grows, Rosie bumps into Bethany, who reveals to Alex about Rosie’s situation. This prompts Alex to return to England to confront Rosie, and despite the initial misunderstanding, they reconnect, with Alex considering becoming Katie's godfather.
While Alex navigates a complicated romantic life with Sally back in Boston, Rosie reintegrates into her profession, working as a maid while nurturing dreams of opening her own hotel. When Alex sends her a ticket to Boston hoping to rekindle their bond, Rosie experiences mixed emotions when she learns about Sally’s pregnancy. A series of misunderstandings ensue; Rosie, wanting her daughter to know her father, reaches out to Greg, who steps back into her life.
Despite their budding family unit, Rosie faces heartbreak as her father passes away, leading to another encounter with Alex. After Rosie and Greg get engaged, a shocking revelation from Sally prompts Alex to declare his love for Rosie through a letter. Unintentionally intercepted by Greg, Rosie remains unaware of Alex’s true feelings while contemplating her own happiness.
Determined to follow her dreams, Rosie begins developing plans for her hotel, while Alex reaches out in search of her happiness. After uncovering Greg's infidelity during a work trip, Rosie realizes it is time to make pivotal changes in her life. Amidst the chaos, she discovers Alex's letter, igniting a wave of hope for rekindled affection.
On the day of Alex’s wedding to Bethany, Rosie gives a moving speech as the best man, pouring her heart out about their shared history and feelings. After an impromptu confrontation regarding the night of her 18th birthday reveals the truth about their feelings, they still find themselves unable to act on their love.
As they bid farewell at the airport, Rosie returns to England, turns her dreams into reality by transforming a manor into a hotel, and hosts an opening party. During the celebration, to her surprise, Alex appears, now without the burdens of the past. The duo shares a moment of reconnection, and as Alex asks her to dance, Rosie responds with “Better late than never,” sealing their long-awaited romance with a passionate kiss.
