Piper, Phoebe and Paige come to the aid of Mylie, a Mermaid-turned-human, when she shows up looking for their help after a pact she made with an evil Sea Hag begins to go drastically wrong. Meanwhile, Phoebe attempts to divorce Cole, until he shows up claiming to want her back, Paige, attempts to juggle her new position as a full fledged Social Worker, as well as being a Charmed One, and Piper must protect her unborn baby, and face her fear of the water demon.