Nick investigates the death of a music teacher who was eaten alive by rats whille locked in his own car. Nick and Hank are led to a expelled music prodigy who has a strange sort for vengeance against those who wronged him.

Nick and the police department goes on the hunt for a mysterious wesen woman who first seduces men and then gruesomely murders them in a "Arachnid"-related death.

When Nick consults him about an erratic murder committed by athletic ex-con Dimitri Skontos, Monroe soon helps him realize that's actually a victim of 'gladiator games' organized by the most bloodthirsty pride of Löwen, a lionesque feral species. Nick spends his anniversary working out the fighters are recruited through a dodgy parole officer and where the arena is hidden. Captain Sean Renard realizes the lucrative fights are becoming too notorious, his royal order to close down the games is ignored, so he turns to a priestly enforcer. Meanwhile first Monroe, then Nick go undercover, only to end up caged and hence in the ring, facing daunting ferals.

Jeweler Sam Bertram was murdered by three European killers, of rival feral species, to steal the rarest Ancient gold coins from his safe. Two are dead, the third steals a uniform from a cop he murders. The coins are found on the elusive Farley Kolt, who is arrested and tells Nick enough to work out he was late aunt Marie's true love. The coins, minted on Zakynthos, with swastika's and lions heads, are poisonous and magically attract great power and utter disaster. Captain Sean Renard keeps them and starts an ambitious 'campaign against crime' amidst dreams of absolute power in Nazi style.

When heavily burned bodies appear, Nick learns of the existence of dragons in the world of wesen and a charismatic fire-dancer tries to spark a flame between herself and Nick, hiding a deadly true agenda.

Nick investigates the robbery of a spice shop, that is owned by wesen, discovering that the thieves were after a dangerous drug.

Nick and Juliette go away for a little R & R. They meet a couple who turns out to be not human. Juliette meets the wife and senses she's being abused. Nick learns the man is a being who abuses women.

Nick is in the trailer trying to find out what the key that he received from his aunt might open. Juliette calls him telling that Hank has invited them to have dinner together with Adalind. Captain Renard is forced by a criminal with a weapon to go with him to meet his cousin, but he reacts and kills them both. When Nick meets Adalind, he recalls that she wanted to kill his aunt and he confronts her near the restrooms. But he is called to investigate the murder of the two men and leaves the restaurant. Meanwhile Renard goes to the house of Catherine and she tells him that her daughter will get the Grimm's key for him. Nick meets with Monroe and Rosalee and Rosalee discovers what Adalind gave to Hank. Will she be capable to prepare an antidote and save his life?

After their face-off with Kimura, Nick finds himself trying to come to grips with the return of his mother, Kelly Burkhardt. A trail of brutal murders unveils a foreign conspiracy to hunt down Nick, leading to several revelations about his past. Meanwhile, Juliette still lies in her mysteriously-induced coma as Rosalee and Monroe work tirelessly to bring her out of it safely. Elsewhere, Hank struggles with the shocking truths he's recently seen and Captain Renard deals with unexpected family issues when his estranged brother comes back into his life.

As Nick and his mother continue to battle the Mauvais Dentes, Monroe and Rosalee fight against the clock to save Juliette. More details are revealed about the source of Juliette's illness, resulting in an epic showdown. Back at the precinct, Hank senses that something is amiss when Nick lands in hot water with the FBI. Meanwhile, Captain Renard continues to reveal himself as he deals with his estranged brother.