On Pine Island off the Maine coast, an aging inn run by the troubled Bart Hunter and his long-suffering wife Sylvia Hunter struggles to stay afloat, with their teenage son Johnny Hunter helping where he can. A reservation arrives from an old acquaintance, Ken Jorgenson, a once modest lifeguard who has since become a wealthy research chemist, who wants to bring his wife and daughter for the summer. Bart hopes to refuse the booking, but Sylvia insists they need the money to keep the inn running, setting the stage for a quietly simmering crisis.
Ken arrives with his wife Helen Jorgenson and teenage daughter Molly Jorgenson. It soon becomes clear that Ken and Sylvia were teen lovers twenty years earlier; their different paths—Ken as a poor college student then, Bart as the son of a wealthy family—helped drive both marriages to unhappiness, yet their feelings endure for the sake of their children. The two begin secretly meeting every night, drawn back to a past they thought they had buried. The island’s night watcher notices the pattern and informs Helen, who initially plans to let the affair unfold so she can secure a larger divorce settlement.
During a hurried weekend away for business, Ken is away and Molly and Johnny, with Ken’s blessing, go sailing around the island. A sudden squall capsizes their boat and strands them on a beach overnight; they are rescued by the Coast Guard the next day. Helen’s suspicions intensify, and when she confronts them with the affair in front of Bart, Ken, Sylvia, and Johnny, a tense confession unfolds. Bart reveals he has long known the truth and offers forgiveness, but Sylvia cannot go back to how things were. The families split, the two marriages crumble, and Molly and Johnny are sent away to boarding schools in different states. The rift between the teens and their parents grows, and Molly and Johnny grow increasingly angry at the adults around them.
Time passes, and Ken and Sylvia, now married, move into a Frank Lloyd Wright–designed beach house to start anew. They invite Molly and Johnny to visit, and during one such visit the young couple sleep together off-camera, discovering a shared love that changes the course of their lives. Molly learns she is pregnant and rushes to plan a life with Johnny. A local justice of the peace suspects they are not of legal marrying age and refuses to marry them. Desperate, the young lovers go to the house of Ken and Sylvia, who stand by them and offer support for their decision to marry. In a hopeful coda, Molly and Johnny, now newly married, return to Pine Island for their honeymoon, ready to build a life together.
