Childhood best friends Rosie and Alex face a lifetime of separation and missed connections that test the strength of their bond. As they transition from adolescence into adulthood, the pair must navigate the physical distance between Dublin and Boston, alongside the complications of separate romantic partners and life-altering personal choices. The narrative unfolds through a non-linear, epistolary framework consisting of letters, emails, instant messages, and notes, which document their evolving perspectives over several decades. They struggle against the constraints of timing and circumstance, constantly questioning whether their deep-seated connection is destined for more than platonic friendship.

The novel is widely recognized for its unique structural approach, which relies entirely on correspondence to convey the passage of time and the depth of the protagonists' bond. Readers often note that the epistolary format creates a distinct sense of intimacy, allowing the audience to witness the characters' growth without traditional omniscient narration. The story is frequently discussed for its focus on the frustration of timing, as the characters repeatedly miss the chance to align their lives. This stylistic choice emphasizes the distance between the two leads, making their eventual reconciliation feel earned through the accumulation of years of shared, albeit written, history.