Grimm began with a retired couple enjoying a restful evening. It gets interrupted by strange sounds outdoors. The husband investigates, but returns mortally injured. Before his wife can call the police, she's attacked and killed as well.
Nick (David Guintoli) and Hank (Russell Hornsby) find evidence and gather fingerprints from the crime scene. The fingerprints lead them to an ex con named Dimitry. They check with the con's parole officer who leads them to the gym. While there they meet Brian, who takes them to the park where he and Dimitry ran. While there, they get a call. Dimitry's car was found.
Near the car they find evidence of a cult. The floor and beams of the warehouse contain carved symbols. Once back at the station, the police captain (Sasha Roiz) translates the Latin, confirming Nick's suspicions about a fight club.
While he waits for information, Nick talks with Juliette (Bitsie Tulloch) about their anniversary dinner that night. Afterwards, as she begins puts laundry away, Juliette uncovers a diamond ring.
Sadly, Brian's run gets cut short when he's chased and taken hostage. He'll be the next creature to die.
Nick goes to do research with Monroe. He uncovers information about a group called the Lowen. They capture creatures and force them to fight one another. Monroe tells Nick he might know how to find the group. He heads to a seedy bar to meet with a bookie named Sully. After showing his Blutbad face, the Mauzhertz (Sully) gives him a meeting place for 6:00 p.m. that evening. Monroe shares the info with Nick who tells him to call as soon as he knows where the fight will take place.
Meanwhile, the captain pays a visit to Dimitry's parole officer, Tanner. The captain chastises the Lowen for involving his paroles in the games. However, Tanner isn't intimidated. He says he'll do what he pleases.
Nick soon discovers that Brian is missing. His truck got abandoned in the same spot as Dimitry's. He tells Hank to check in with Tanner while he follows Monroe.
The Blutbad calls Nick to say no one showed up, but while he's talking, he gets captured and taken away. All Nick knows is that his friend is near somewhere close to where Dimitry and Brian went missing. He heads there, hoping to find Monroe.
At the fight club, Tanner tortures Monroe with electricity to get him ready to fight. In the ring, Brian has already been defeated and killed.
As Nick arrives at the fight site, he gets a call from Hank. He says he's found enough stuff at Tanner's house to implicate him in the murders. Nick asks for backup, perhaps too late. Monroe is already losing his fight.
The Grimm threatens to kill Tanner if he doesn't let Monroe go. However, he's quickly surrounded by creatures who will attack if he harms the Lowen. So instead, he offers to fight for both Monroe's and his freedom. It's an offer Tanner can't refuse.
Elsewhere, the captain attends a confessional and tells the "priest" about his problem with Tanner. Then, he mysteriously asks the priest to "bring his wrath."
Nick wins the fight, but the creatures won't let him go free. Luckily, the police arrive just in time. The Lowen temporarily escape, but back at his office, he's attacked and killed by something unseen.
At home, Juliette stares at the ruined dinner. Nick calls to apologize for being late. She says it's okay but then stares at the ring with sadness and longing.