With Nick back in his house with Juliette and Rosalee moving in with Monroe, all seems right in the world. But across town, the Columbia River is about to become a watery grave.
Two couples sing loudly and off-key. The first couple, Dan and Anna, splash off into the water and leave the other couple, Jake and Sarah, alone on shore. Excusing herself to grab a beer, Sarah veers behind the bushes to chide her younger sister for spying. Her deaf sister Elly begs her "not him" because she loves Jake. Sarah dismisses Elly and strides back to Jake, confidently taking off her swim top and plunging into the water. As things get steamy in the lake, Elly pockets Jake's phone to scroll through his photos.
After Jake and Sarah finish, Dan screams and disappears underwater. The girls evacuate the lake while Jake fishes for his friend. Suddenly, a webbed hand grabs his leg and pulls him down, down, down... An unconscious Jake floats to the top of the water and is dragged out by another pair of webbed hands - Elly. When Jake comes to, he hazily sees Elly in Wesen form before she darts off.
The next day, Dan's lifeless body is drawn from the river. Nick and Hank break the news to Jake since he filed the police report. What happened and how did he escape? According to Jake, the two sisters, Sarah and Anna, randomly showed up at the boys' fishing site, had sex with them, then vacated when trouble arose. Jake vaguely remembers another girl saving him, but he'd never seen her before.
While visiting the fishing site to jog Jake's memory, Nick hears Elly rustling the surrounding bushes. He races after her but loses her once she jumps into the river - she's clearly Wesen, but Nick's never seen her kind before. When he describes her features to Monroe and Rosalee, Rosalee suggests she's a Naiad. Aunt Marie's trailer proves Rosalee correct - Elly's a Naiad, a rare Wesen that must live by water or die from dehydration. The species is also noted for its nearly unbreakable familial bonds.
Following the signal from Jake's missing phone brings them to a house in a nearby marina. Of course, it's where Elly lives, but she spots the police before they discover her. Abel, her Naiad father, stops Nick and Hank's investigation, claiming to know nothing about a drowning or a missing phone... However, a quick call to Jake's cell from Officer Wu gives them probable cause to search the house. Nick quickly locates the cell, while Jake confronts newly arrived Sarah and Anna outside. Both girls deny knowing him, but the police aren't fooled. As Sarah and Anna are arrested, their "neighbors" Dominic and Jesse put up a stink until they're threatened with jail time. When Nick and Hank leave, fundamental Naiads Dominic and Jesse bully Abel, claiming Elly must be "cut" for her interference.
Dehydration takes rapid effect as Sarah's and Anna's skin peels during their interrogations. Despite certain and imminent death, neither girl talks. Renard shrugs off the girls' impending doom because he won't make exceptions for Wesen. To save his daughters, Abel confesses he killed Dan. He claims he believed the boys were assaulting Sarah and Anna, so he took necessary action. Nick and Hank see through his thinly veiled rescue attempt and poke massive holes in his story.
Meanwhile, Jake discovers an open window at his apartment and is surprised by his savior Elly. Determined to "explain" herself, the deaf girl leads him out onto the pool deck and jumps in the water to reveal her Naiad form. Marveling over the sight, Jake is caught unawares by Dominic and Jesse who beat him before heaving a faintly screaming Elly over their shoulders.
Sarah immediately breaks when she discovers her father has confessed, and she admits Dominic and Jesse killed Dan, not her father! She and Anna had arranged marriages with Dominic and Jesse, who migrated from Alaska six months ago. Starting a Naiad family is unconventional since Naiad men are born sterile. Children can only be conceived in water by male humans and female Naiads. Naturally, this leads to a certain amount of jealousy among Naiad men. Dominic and Jesse might say killing Dan kept with tradition, but Sarah knows it was a revenge killing.
Jake's SOS call sends Nick and Hank to the marina just in time to save Elly from having her webbed fingers and toes sliced by the two vengeful Naiads. Instead, they Elly drop her in the water with an anchor tied around her feet. Nick saves her, but his heroism is suspicious since his dive required more breath than a normal human has...
Meanwhile, Captain Renard discovers he might be the father of the unknown royal baby. After all, the Sacher hotel room belonged to Adalind.
Juliette checks the email account she shares with Nick and is ecstatic to discover he has a clean bill of health from the doctor. However, her joy quickly dissipates when she reads an email signed, "Love you, M." Who is M?!
A watery revenge killing leads Nick to discover a new Wesen species.