Fifteen years earlier, Percy begins the summer at the cottage by joining Sam's new running routine. Sam approaches training with unexpected seriousness, but Percy struggles through the workouts and is far more distracted by how much Sam has changed physically. After one miserable run, Percy strips down to a thong before jumping in the lake, and Sam's flustered reaction makes their mutual attraction impossible to ignore, even if neither of them names it directly.

When Charlie suggests distance swimming instead, Percy eagerly changes course. Sam helps her train every day, buys practical gear with her in town, and stays close during practice, often coaching her from the water or rowing beside her. Their closeness keeps producing intimate moments, including a small but charged incident when Sam casually licks her melting ice cream, leaving Percy intensely aware of him.

As Percy prepares to swim across the lake by the end of August, Sue reveals that Sam has been lifting weights because he wants to be strong enough to pull Percy out if anything goes wrong. On the day of the swim, Sam is more anxious than Percy, but Percy reassures him and completes the crossing through pain and exhaustion. Sam is overwhelmed with pride and relief when she reaches shore, and back at the house Sue feeds everyone breakfast and tells Percy she is proud of her as if she were her own daughter, making the achievement feel like the perfect end to the summer.

After summer, Percy and Sam keep writing. At New Year's, Sam joins Percy's family for a themed dinner, and when they are alone afterward, he asks about Delilah's cousin Mason, jokingly calling him Buckley. Percy admits they have been on two dates and says she does not think boys want to kiss her; Sam pushes back, tells her that is not true, and adds that he does care, revealing jealousy and emotional investment he usually keeps hidden.

In January, Mason's interest becomes explicit. He kisses Percy after a hockey game, then at his lavish eighteenth birthday party he gives her a bracelet and asks her to be his girlfriend. Although Mason is kind and attractive, Percy cannot return his feelings fully: when they kiss more deeply in private, she imagines Sam instead and pulls away. Afterward, Percy tells Sam by email that she likes someone else more than Mason, and Sam replies that she already has her answer, leaving Percy and Sam's unresolved feelings even more pronounced.