Bristol Cove is a coastal town known for its legend of once being home to mermaids. When the arrival of a mysterious girl proves this to be true, the battle between man and sea takes a vicious turn.

While on the North Star, Ben discovers a strange device in the water that could lead to Chris.

Dale and the local police's investigation starts pointing directly at Ryn. Xander and Calvin's search for Chris heats up when they find Donna, the nurse who works at the military bunker.

When the military facility is on high alert, Chris escapes along with a traumatized "nurse."

Old friends return to Bristol Cove. Ben investigates some suspicious overfishing. Decker ramps up his efforts to find his escaped mermaid and shows the true extent of his influence.

Donna and Ryn clash over their feelings about humans. Xander, Chris and Decker race to capture their own mermaid.