A ship is at set and they catch soemthing, it turns out to be a mermaid but before they cna do anything it is taken away from them by possibly the government. They also take one of their men that was injured from a bit by the creature they found.
In Bristol Cove there is a mermaid days festival going on and Ben and Maddie are introduced. Ben meets Xander at the bar later who tells him what happened at sea and what they think they saw. Ben doesn't believe them and thinks they should let it go.
At a government secret location the mermaid is being kept in a tank. Ben and his mother talk about how his father is upset with him about not being present at a public event to represent their family.
A shark head is found in the water which gets Ben thinking about what Xander described haunting the ocean. While driving back home he runs into a strnager, who we know as Ryn. She faints and he takes her with him so the doctor can inspect her. She can't seem to talk to him, except when she starts to sing a song that he gets caught up in.
Ryn is gone when the doctor shows up though and she ends up staying on a wrecked boat on land. She walks by a place in town where a woman recognizes who she is. Ben drives by in that moment but Ryn escapes him again. Ryn has flashbacks that confirm that she is a mermaid and she saw the other mermaid get caught and taken away.
Maddie is having dinner with her dad when during grace he brings up their mom. Maddie mentions that maybe they should stop mentioning her since they haven't heard from her in eight months.
Ryn adopts her name and then comes across a man that offers her a ride. In the car he tries to force himself on her and she kills him by throwing him through the windshield of his car.
She then finds her way back to Ben at the marine research center where they end up together into the night waiting for a doctor.
Ryn gets worse and worse because her body needs the ocean and she ends up crawling into it off the dock and becoming her mermaid self. Ben jumps into the water to check what is going on and gets attacked by her. He escapes and goes to see the woman from before who told him when he was a kid about mermaids.
She tells him the truth about how humans never get that they are prey to mermaids and his ancestors lost their minds slaughtering mermaids in cold blood.
Ben goes to see Xander and asks to go with them on a trip out to sea. While out there they seem to find something in the ocean and Ben is conflicted about hurting whatever it is because of what he learned.