Donna is on the brink of death so Decker is called in to help her but Maddie isn't happy. She thinks Ben is changing and they aren't in these things together anymore. Donna gets better but then gets worse from an infection and dies in Ryn's arms.
Helen takes Ryn, Ben, and Maddie to a burial ground that belongs to her family of mermaids that were human, they were between worlds. She talks a bit about the history and reveals that she is related to Ben. Ryn agrees to bury Donna there and then herself when the time comes.
Meanwhile Calvin finds out about Xander sleeping with his girlfriend when he suggests they move in together. They have a fight but figure it out by the end.
Decker is struggling because he keeps thinking about the siren song. Helen warns them all that it can cause people who hear it to become obsessed and Decker goes out to sea and drowns himself.
Maddie finds out that Ben asked Ryn to sing him a song and that is how they track him down when he is about to pass out underwater because he thought he heard Ryn calling him.
Maddie and Ben decided to part because Ben makes decisions that Maddie doesn't understand. Ryn then visits Ben and says she will keep her distance from him because she doesn't want to hurt him.
Maddie and her father are having dinner when they get a call from her mother.