Arthur Peabody, a middle-aged man in the throes of a midlife crisis, vacationing in the Caribbean with his wife Polly, unexpectedly stumbles upon something extraordinary while out fishing. On what begins as an ordinary morning, Arthur hears mysterious singing that leads him to reel in an enchanting and beautiful mermaid. Naming her Lenore, he brings her back to their hotel and attempts to keep her presence a secret. Arthur endeavors to learn more about the mermaid, who cannot speak but displays childlike curiosity and innocence. This newfound secret and the magical allure of Lenore ignite a sense of youthful adventure and romance in Arthur, something he's been missing in his routine life.

As Arthur spends more time with Lenore, balancing his interaction with her and the suspicions of his wife Polly becomes increasingly difficult. His eccentric behavior raises Polly’s awareness, challenging their marriage and culminating in confrontations that test Arthur's resolve and morals. Meanwhile, Lenore's presence disrupts the tranquil environment, leading to misunderstandings and comedic situations that complicate Arthur’s attempt to keep her hidden. The mysterious allure of the mermaid propels Arthur into a series of misadventures that reflect on themes of wonder, renewal, and the search for meaning amidst life's mundanities.