Matt returns to his childhood town with his daughter Ryan, to look after his father Art, who's health is failing. Art refuses to take it easy, and Matt goes fishing with him. Meanwhile Ryan explores the town and finds the docks. Ryan notices someone trashing underwater, and pulls the rope closer, only to see someone trapped in the net. Ryan cuts the rope and releases the girl trapped there, and she swims away. Ryan then notices that her locket is missing, and is really depressed by it, since it was the only thing left of her mother. When Ryan asks about why one would cast a net at the docks, Art tells her about mermaids. Ryan starts dreaming about swimming with mermaids while submerged in a bath, and wakes up ten minutes later. The next day, Ryan goes looking for her locket, and meets Coral, the mermaid she rescued. Coral had found Ryan's locket and gave it back to her, but she had to return to the sea, as her mother was calling her. Matt meets his old fling, Jenna, while dining out with his daughter. Art wakes Ryan up at 5am, and puts her to work, repairing the fishing nets. Coral visits Ryan while there, and confesses shredding the nets. They agree to meet later, after Ryan has finished repairing the nets. Later they are swimming in the sea, and after admitting they are both curious about the world the other lives in.They hatch a plan to show each other the world they live in. Ryan gets stung by a jellyfish,and Coral shows off her healing power. The next day Art takes Matt to a fishing trip for couple of days. Matt leaves Ryan in the care of Jenna. Ryan hangs out with Coral on land, and they go shopping and diving. The next day Coral takes her to see her home hidden magically on a deserted island. Ryan meets other mermaids living there, and Coral's grandmother, Thaleia, the queen. At first they forbid Ryan to return to home, but after hearing who her grandfather is, the queen allows her to return. Ryan and Coral switch amulets to remember each-other by. Art gets a heart attack while on the fishing trip, and they have to cut it short. Ryan learn that her grandfather is in hospital, and visits him there. Art recognizes the amulet and asks her to show where the mermaids live. Ryan refuses, but after Art explains that he fell in love with a mermaid, and that Matt is the mermaid's son, she agrees to take him. On the way to the island they rescue Coral, who is stuck in a net on the sea. And Art suffers another heart attack. Ryan and Coral suggest to go to the island, so the mermaids can heal him. At fist the mermaids are afraid to show themselves, but after Matt arrives with Jenna, the mermaids show up. Thaleia reveals herself to be Matt's mother to Matt. The mermaids from that day on change their laws to never show themselves to humans.