Meredith and Marina's lives have been flipped upside down. When the translucent skirt that straight-A-student Meredith finds hidden on the beach gets wet, it transforms her legs into a mermaid tail. Despite the evidence in front of her, she insists that becoming a mythical creature isn't scientifically possible. Marina is allowed to experience one day per moon cycle among the humans. After hiding her tail on the beach that morning, she takes her first timid steps on land. When she returns at sunset to find it missing, she is left stranded and alone. For the first time in her life, Meredith doesn't have all the answers. As she searches for a way to return to normal before the next full moon, she makes waves among the school of mermaids. Meanwhile, Marina uncovers information about her past, and for the first time in her life, she must stand on her own two feet and take the lead on her own adventure. As Meredith senses her human side slipping away, a forbidden way to change back entices her. But it comes with a consequence: Marina wouldn't be able to return to the ocean.