Cat and Vincent will be forced to work together as a couple in ways they never have before if they hope to defeat their destined foe in Liam, who will do everything he can to expose Vincent as a beast in order to destroy their love.

Liam's history spurs Cat to dig into the Ellingsworth family's past; Vincent and Cat realize that they may need their secret weapon to defeat Liam; and JT develops a theory on how to save himself.

In a surprising turn of events, Vincent agrees to help Liam on a mission to retrieve evidence that could expose them both. Meanwhile, a suspicious Cat discovers something about Liam's family history and must warn Vincent before it is too late.

Cat and Vincent set out to capture Liam; and come across information that could help them do just that. In other events, JT and Tess work to confirm that JT is serum-free.

Vincent, Tess and Heather try to boost Cat's spirits after the events of her big day leave her doubtful that they can defeat the threat.

Cat's thrilled about her pending nuptials, but is distracted by the notion that the rash of superhuman experiments could ruin her big day, so Vincent, JT and Tess do their best to keep her eyes on the prize.

An assassin targets Cat and Vincent, who attempt to lure him out of town and away from their loved ones by pretending that they're going to elope; and JT, who's tired of being under police protection, asks Tess to help him track down a lead. A distracted Heather, meanwhile, tries to focus on her new job.

JT throws Vincent a surprise bachelor party. Unfortunately, it is interrupted by a mysterious sniper with his sights set on putting a stop to Vincent and Cat's interference with the super human experiments. Cat's family comes to town for a bridal shower arranged by Heather but Cat is distracted by having to visit her imprisoned father (guest star Ted Whittall) to ask for his help.

Cat is assigned to a new partner; and comes to suspect that Bob and Carol are working to separate her from Vincent. In other events, JT learns that his newfound healing ability comes with a high price.

Cat and Vincent take their relationship to a couples counselor (Saul Rubinek) when Cat dives into her work and suspects a wealthy transplant recipient of being involved with the mysterious experiments on innocent people in the city.

When two married agents (guest stars Alan Van Sprang and Natasha Henstridge) from the FBI take over an investigation, Cat and Vincent hope they are seeing a future version of themselves but fear their work schedules are jeopardizing their relationship. When the two couples bond and team up on the case, a mysterious coincidence surfaces that raises suspicions. JT investigates whether he has super powers of his own while Tess considers a career move. Meanwhile, Heather offers up an interesting option to make her big sister's life a little easier.

Vincent moves in with Cat which forces them both to address the rapid changes in their relationship while they investigate a shocking new lead in their hunt to track down who is behind the super-human experiments. JT tries to impress Tess' brothers and discovers that there may be some side effects to the medicine he received from Agent Thomas. Meanwhile, Heather returns to town with a surprise announcement.

With the happy couple seeming to be enjoying an ordinary life, Vincent thinks the timing might finally be right to pop the question until Cat stumbles on an extraordinary case with the FBI involving somebody who goes on a superhuman rampage. Though reluctant to engage with the case for fear of losing the normality he has found, Vincent finds himself with no other options when Cat is injured after discovering that someone is drugging innocent victims and transforming them into super-humans. Meanwhile, Tess accuses JT of stalling his recovery as he tries to understand how he's still alive.