Note the themes below:
Plot
In 1964, eight-year-old Allen Bauer and his family are taking a boat tour at Cape Cod, Massachusetts. Allen is fascinated by something below the surface and jumps overboard. In the ocean, he encounters a young girl and inexplicably finds himself able to breathe underwater. However, Allen is rescued and pulled back to the surface, and the two are separated. Since no one else saw the girl, Allen comes to believe the encounter was a hallucination.
Twenty years later, Allen is now co-owner of a wholesale fruit and vegetable business in NYC with his womanizing older brother Freddie. Throughout the years, Allen's relationships have failed as he subconsciously seeks the connection he feels other couples enjoy. Depressed after his latest breakup, Allen returns to Cape Cod, where he encounters eccentric scientist Walter Kornbluth on a diving expedition.
When his motorboat restarts, Allen falls into the sea and is knocked unconscious; his wallet drops onto the coral below. He later wakes up on a beach, in the presence of a beautiful young woman who is unable to talk. After kissing Allen, the woman dives into the sea, where she transforms into a mermaid. While swimming underwater, she is sighted by Kornbluth.
The mermaid finds Allen's wallet, and uses a sunken ship's charts to locate NYC She comes ashore naked at the Statue of Liberty and is arrested for indecent exposure. Using information from the wallet, the police contact him and she is released into his care. The mermaid learns how to speak English from watching television, and is eager to explore the city.
Unable to say her real name in human language, she selects "Madison" from a Madison Avenue sign. She tells Allen that she will be in New York for "six fun-filled days until the moon is full", unable to return home if she stays any longer.
Despite Madison's occasionally outlandish behavior, she and Allen fall in love. He proposes to her, but she declines and runs away. After some contemplation, Madison returns to Allen and agrees to marry him, with the added promise of telling him the truth about herself after an upcoming dignitary dinner to welcome the President.
Meanwhile, Kornbluth, realizing that Madison is the mermaid he encountered, pursues the couple, trying to expose her by splashing her with water. His first attempts are unsuccessful, and Kornbluth ends up with multiple injuries. Finally, he infiltrates the dignitary dinner, soaking Madison with a hose and successfully unmasking her identity.
Madison is seized by government agents and taken to a secret lab, headed by Kornbluth's cold-hearted rival Dr. Ross for examination. As she withers away in captivity, Kornbluth learns that the scientists are planning to dissect her, causing him to regret his actions.
Allen is shocked by Madison's secret and rejects her, but when he voices his disillusionment to his brother, Freddie lashes out at him, reminding him how happy he was with her. Realizing that he still loves Madison, Allen confronts the guilt-ridden Kornbluth, who having been rejected by his colleagues, agrees to help rescue her.
Impersonating Swedish scientists, Allen, Freddie, and Kornbluth enter the lab and smuggle Madison outside. Freddie stays behind to be arrested in Allen's place, while Kornbluth unsuccessfully tries to stop military troops from catching the couple. Despite being pursued, Allen and Madison make it to New York Harbor.
Madison tells Allen that he can survive underwater as long as he is with her, prompting Allen to realize that she was the girl he had met underwater as a child. She warns him that if he comes to live in the sea, he cannot return to land.
Madison dives into the water as the troops close in on them. When they attempt to arrest Allen, he jumps in after her, but starts to drown. She kisses him, gifting him the ability to swim and breathe underwater. Frogmen try to recapture them, but the couple fight them off and escape. Allen discards his jacket, forsaking his miserable life on the surface and the couple happily swims toward what appears to be an underwater kingdom.
