"Splash" is a very funny and entertaining romantic comedy starring Tom Hanks and Daryl Hannah, who plays the role of a mermaid. Although the plot is pretty ridiculous, the acting is quite good. John Candy also has a funny role as Freddie, the rather perverted brother of Allen (Hanks), who works for Penthouse magazine.
In 1964, as an 8-year-old boy, Allen takes a boat trip with his family in Cape Cod. There he encounters a mermaid about his age in the water. He jumps after her and swims underwater with her a bit before the boat crew save him.
Now 20 years later (in 1984, the year the film was made), Allen lives in New York City where he sells fruit. Allen is depressed because his girlfriend has left him. He complains to his brother Freddie that he seems to be incapable of being in love. He decides to take a trip to Cape Cod. There on an island he meets the same mermaid he'd met as a kid, but doesn't realize she's a mermaid since she doesn't show him her tail, which she only has when she's in the water. Allen and the mermaid make love on the island. A biologist named Dr. Walter Kornbluth sees her in the water with a tail and is determined to prove his finding to the rest of the world.
Allen leaves his wallet on the island by accident. The mermaid looks at his ID and sees that he's from New York City. Then she finds a map on an old abandoned boat underwater , looks up New York City, and swims there with Allen's wallet.
The mermaid shows up naked in front of the Statue of Liberty, where she is arrested for indecent exposure. The police call Allen, whose number they find from the information on his ID. Allen rushes to the police office to pick up the mermaid. He takes her to his apartment and she borrows his clothes.
While he goes to work, the mermaid goes to Bloomingdale's and uses Allen's money to buy some new clothes. She also learns English by watching TV from a group of TV sets on display. After work, Allen learns from the concierge that she went to Bloomingdale's and picks her up there. He is surprised to learn that she can now speak English, just from having watched a few hours of TV. He asks her name, but when trying to answer, she ends up breaking the TV sets with a loud shriek.
Allen is very much in love with her but knows practically nothing about her, least of all that she's a mermaid. He gives her the name Madison, which she chooses while walking down Madison Avenue. Madison tells Allen that she can only spend six days with her, but won't tell him why or reveal her true identity.
A few days later, while ice skating with Madison in front of the Rockefeller Center, he decides to propose to her, but she tells him no flat out. Allen becomes very upset, so she finally changes her mind. She realizes that she must reveal her secret to him in time.
Eventually the day comes when Madison must reveal her true identity. Dr. Kornbluth has been trying for the last several days to get her wet in order to prove that she's a mermaid. Finally he succeeds after a presidential dinner. Several onlookers are stunned when they see her tail, including Allen. Madison and Allen are taken to a top secret research lab where they're investigated. Allen is placed in a tank for 12 hours, but since no changes in him are observed, he is released.
Now Allen is heartbroken once again, since he realizes that he'd fallen in love with a mermaid. His brother Freddy convinces him that what he has with her is very special and to go after her again. Thus he gets Dr. Kornbluth to sneak him and Freddy into the lab to rescue her. They manage to get her out, though the feds still go after her.
Eventually Madison tells Allen that they can live together underwater, but that he can never return. After some deliberation he agrees and they jump into the harbor together. They fight off some divers who go after them and swim together to an underwater city.