Cat and Vincent are able to focus on their love, finally free from Muirfield, manhunts, and beasts. However, that love will be tested as new and more dangerous threats await. When they learn that innocent victims are being used to test the boundaries of the impossible, Cat and Vincent will uncover a deeper conspiracy, one which Vincent can fight only by turning to the beast he carries within. But unleashing the beast will risk Vincent's life - along with his and Cat's love - as they struggle to build a normal life together.
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.
When two married agents (Alan Van Sprang and Natasha Henstridge) from the FBI take over an investigation, Catherine 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.
Cat and Vincent take their relationship to a couples counselor 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 Cat is assigned to a new partner by Tess, she begins to suspect that Bob and Carol have re-surfaced to separate her from Vincent and hunt them both down. Meanwhile, a transformed JT discovers that his newfound healing ability comes at a terrible cost.
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 to ask for his help.
When an unstoppable assassin hunts down Cat and Vincent, they attempt to lure him out of town and away from their loved ones with the ruse that they plan to elope. Meanwhile, tired of being under police protection, JT asks Tess to help him track down a lead while a distracted Heather attempts to focus at her new job.
Cat is excited to finally walk down the aisle with Vincent, but can't help worry that the rash of superhuman experiments will ruin her destined moment. Vincent, JT and Tess are determined to do anything they can to keep Cat focused on her big day before all hell breaks loose.
After a force more powerful than anything they have ever encountered ruined Cat and Vincent's big day, Cat becomes convinced they are in over their heads and begins to doubt their ability to stop the threat, forcing Vincent, JT, Tess and Heather to band together in hopes of convincing Cat of her greater purpose.
When Cat and Vincent make it their mission to capture Liam, they find information about a Patient X that they might be able to use to help them find him. Meanwhile, JT and Tess work to confirm that JT is serum free.
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.
Liam's shocking history makes Cat delve deeper into the Ellingsworth family, where she learns of their tragic past. Vincent and Cat realize that they will need to use their secret weapon if they want to take Liam down. Meanwhile, JT has a theory about how to save himself but it involves none other than Liam.
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.