A doctor in South America is bitten by a mosquito before returning home to Portland. After reuniting with his wife, he becomes ill. Leaving the house for a walk, he transforms into a monster and fatally attacks an early-morning dog walker. The man who finds the bodies tells Nick and Hank that the Chupacabra did it.
Nick and Hank look into the legend of the Chupacabra and learn that the creature is actually the result of an infection or plague transforming other Wesen.
The doctor transforms a second time, attacking one of his own friends before fleeing. Nick and Hank interview the doctor's wife, who pleads with them to help her husband.
Sgt. Wu confronts Nick and Hank about all the secrets they've been keeping from him. Before they can explain very far, the doctor returns and transforms yet again. Wu is shocked and horrified to see the creature and leaves in a hurry after Nick and Hank secure the doctor in handcuffs.
They take the doctor and his wife to Rosalee's spice shop, where Rosalee mixes a cure for the doctor's ailment. But then the doctor's wife transforms, too. The doctor grabs the cure and gives it to his wife, forcing the cops to kill him when he transforms one last time.
Monroe is concerned about all the trouble he and Rosalee have been receiving from the Wesenrein over their marriage. Rosalee tries to relieve his worries until they leave on their honeymoon, though she is understandably upset after receiving a threatening phone call and discovering a dead fox hanging outside her shop.
Nick leaves a cop outside their home for protection, but it turns out the cop is Wesenrein, too. They kidnap Monroe.
Juliette continues to have strange headaches and nausea. It all comes to a head when she transforms into a Hexenbiest in front of her mirror at home. She screams in horror and terror at her reflection.