"The beasts were loosed into the arena, and among them, a beast of huge bulk and ferocious aspect. Then the slave was cast in."
A creature in human form is tracking a trail through the woods. Once he gets the scent, he and two others of his kind ride on horseback in pursuit of their target.
The Wellers, Ed and Lois are at their cabin in the woods and preparing steaks for supper. They hear their dog Ruby barking outside and Ed goes to investigate. A skalenzahne leaps out of the brush at him and attacks him, and Ed manages to stagger back to the cabin. His wife drops the raw steaks in horror at the sight of her disemboweled husband, and the creature, Dimitri, leaps past the body and attacks her. It then grabs the raw steaks, but hears the horses outside. The skalenzahne tries to run but the three hunters quickly lasso him and haul him away.
The next day, Nick and Hank are called to the cabin to check out the crime scene. Franco confirms that the CSU techs have found lots of prints, as well as the half-chewed steak. Outside the detectives find horse tracks and follow the trail, noting that the Wellers didn't own horses. Nick finds drag marks as well as a strap from what appears to be a medieval saddle.
When Nick gets back to the station, Juliette calls to see what he wants for their anniversary dinner. Nick goes over the crime scene photos and vaguely tells her to make whatever she wants. After Juliette hangs up, she puts away the laundry and finds Nick's engagement ring for her, hidden in his sock drawer. Meanwhile, the officers get a match off one of the fingerprints. It belongs to Dimitri Skontos, a minor crook who recently served a year for possession. While Nick puts out an APB on Dimitri's car, Hank sets up a meeting with Leo Taymor, Dimitri's parole officer. Leo is surprised to hear that Dimitri is wanted for murder and assures them the ex-con is no prisoner. However, Dimitri didn't show up for his weekly meeting and Leo admits that he might be wrong about Dimitri's character. When the officers ask if Dimitri had any family, Leo directs them to Dimitri's uncle, Gus Pappas.
The detectives go to the gym where Gus works, and he admits that he hasn't seen Dimitri in a week, but figures his nephew can take care of himself. They ask about Dimitri's friends and Gus directs them to Brian Cooney, a local boxer. Nick realizes that he's a dickfellig and Brian realizes he's facing a Grimm. The boxer prepares to fight, but Nick assures him that he doesn't want any trouble and Brian apologizes. Brian says that the last time he saw Dimitri was a couple of weeks ago when they went running in Forest Park together, and invites Nick and Hank along as he goes on his run. As Brian runs through the forest, the detectives trail behind and Nick confirms that they're only six miles from the Wellers' cabin. They find horse manure along the trail and get a call from an abandoned warehouse. Dimitri's car has been found there and Franco is containing the crime scene. As Nick and Hank go back to their car, Brian continues running... unaware that the creatures from the other night are on horseback and following him.
At the warehouse, Franco tells the detectives that thieves stripped the car and there's no signs of blood or skin. Nick spots a man watching them and they go after him, but he loses them in the warehouse. They spot blood on the wall and Franco goes back to call it in. The officers hear a radio signal from Franco and investigate, and he tells them that the man attacked him with a weapon, knocked him down, and run off.
In the forest, the three horseman run Brian to ground, rope him, and drag him away.
Hank and Nick return to the station and discover that the CSU techs found medieval weaponry at the warehouse. The officers figure there some kind of fight club was held there and also receive a photo of a Latin phrase inscribed on the floor. They have Renard translate it, and he explains that it's praise to knowing death and inflicting it on others. Nick and Hank give him a report and confirm that the blood at the warehouse was Dimitri's, and they found a strap similar to the one near the Weller cabin. After a moment, Renard tells them to continue with the investigation.
Later, Renard cuts off Leo in the latter's parking garage and tells him that his arena fights are out of control and that they have an agreement. Leo assures the captain that Dimitri is now back under control, but Renard warns him that Dimitri wasn't on the list that he gave Leo, and that it's his rules in his city. Unimpressed, Leo says that the cheap street thugs that Renard has been "giving" him don't cut it and that Dimitri has won six straight fights and has big money riding on him. Renard tells him to shut it down but Leo says that his royal status isn't what it was. The chief draws his gun and tells Leo to clean it up, but Leo warns him that since they're partners, Renard should be careful how he treats him.
Nick goes to his aunt's trailer to examine her weaponry and also locates a harness with the same straps as the ones he found. He also checks her journals and finds a reference to lowen. Eddie comes in at Nick's invitation and assures his friend that he wasn't followed. He tells Nick that the lowen were enslaved by the Romans for their gladiator fights and now run their own fights, fueled by generations of enslavement. Eddie doesn't know anything about modern day fighting and warns that the lowen keep to themselves, but offers to check with a friend of a friend and get an address.
In the arena, Dimitri fights Brian and defeats him. He turns to Leo, the ruling lowen, and the parole officer gives Brian the thumbs down as the crowd cheers.
The next day, Eddie meets a bookie, Sauly, at a bar. They both reveal that they're wesen and Eddie wants to place bets on the lowen games. Sauly is reluctant to help but finally provides an address to go to where someone will take Eddie to the games. Outside, Eddie calls Nick at the station, who says that he'll take it from there. Meanwhile, Hank has confirmed that the warehouse is in probate and no one owns it, making it the perfect place to hold an underground fight club. Renard tells the detectives that Brian has turned up missing, his car was dumped at the warehouse, and that Leo was also his parole officer. They figure they should talk to Leo and Renard suggests that they talk to him at home rather than the parole office.
That night, Nick goes to the warehouse to meet Franco and check on Brian's car, but it doesn't yield anything useful. Meanwhile, Eddie goes to the rendezvous spot and calls Nick. When he says that a truck with a horse trailer is pulling up, Nick figures that it's a trap and tells him to get out. However, one of the lowen hunters ambushes Eddie and knocks him out, drags him into the truck, and smashes his cell phone. One of them stays behind to get rid of Eddie's car, and Nick arrives in time to capture him and get the address out of him.
The lowen take Eddie to the barn where the new fights are being held and put him in a cage along with the other prisoners. When Leo comes in, Eddie tries to convince him he's not an aggressive blutbad, but Leo isn't convinced. After the lowen leave, Eddie sees Dimitri in the next cage. The skalenzahne is in human form and has a nail stuck in his hand. Eddie offers to pull it out and Dimitri reluctantly lets him do so, and then eats the meat that the lowen are tossing into the cages. Eddie looks at it in disgust when he realizes that it's human flesh.
After the meal, Leo comes back to collect Eddie and Dimitri for the next fight. When Eddie refuses, one of the lowen uses a taser to shock Eddie repeatedly. He transforms into his bestial form and lunges at them, and Leo is satisfied that he'll put on a good performance.
Hank goes to see Leo at his home in the country but no one answers the door. When the detective hears a horse neighing nearby, he goes to investigate and finds a barn filled with medieval weaponry. When Nick calls to tell Hank that he has the address of the fight, Hank tells him what he's found and figures that Leo is there. His partner tells him to bring up and goes to investigate.
Leo and his hunters throw Eddie into the fight cage with Dimitri, who savagely attacks the blutbad with the weapons at hand. Eddie tries to defend himself but is soon overwhelmed. Nick, who has snuck in, gets next to Leo and puts a gun to his head. However, when he orders him to surrender, Leo refuses, saying that he'll die with honor and then the spectators will rip Nick apart. He insists that only the winner can leave and Nick takes him up on the challenge, offering to fight in Eddie's place. Leo smiles in triumph and tells the crowd the news, and warns Nick that he has to leave his gun behind. When Nick wonders how he can trust him, Leo points out that he doesn't have a choice.
Renard goes to a local church and enters the confession booth. He tells the priest on the other side that there is a fellow "parishioner" that no longer fears the sword, and that redemption isn't a possibility. Renard asks the priest for his wrath, and the priest ask Renard to give him a few minutes to change.
Nick tells Eddie that he's trying to buy time until Hank can arrive, and Eddie tells him to draw on the memory of his Grimm ancestors to fight back. Nick finally manages to knock Dimitri down and hold a sword to his throat, but says that he doesn't want to hurt him. Leo sends one of his hunters in to take care of things and Eddie knocks him out, and Hank and the officers burst in. Leo slips away in the confusion while the police capture the spectators.
Juliette is sitting at home at the dinner table, checking her watch. She finally blows out the candle and takes out Nick's engagement ring. He calls to tell her that he's on his way and she vaguely assures him that it's not too late. When he says that he loves her, she hangs up and puts the engagement ring back.
Leo goes to the hay loft to get his secret stash of the profits. Renard comes in and Leo promises to make him pay. Renard tells him that someone is going to pay, and the "priest" lunges out of the shadows and tears Leo apart while Renard walks away.