I am impelled not to squeak like a grateful and frightened mouse, but to roar...
In a parking garage, a man tries to hide as someone calls out and says that he's going to get what he deserves, and that there's nowhere to hide. The man finally locates the person who is hounding, him, ducks around the corner to attack him... and ends ups stabbed in the throat with a screwdriver. His killer, who sees the dead man as a different, much older man, drags the corpse outside and tosses it into a dumpster.
The next day, sanitation workers unwittingly take the body away. Nick, Hank, and Wu are called in when the workers finally find the body in their truck. They confirm that the dead man is Leonard Lewis Drake, who had three domestic assault charges filed against him. The officers go to the dumpster and follow the blood to the apartment parking garage. The landlord, Forest McCleary, says that Leonard was fighting with Mason Snyder, a lawyer he lives next door, and that Leonard lives with his girlfriend, Natalie Haverstraw. Hank and Nick go to Leonard's apartment and discover that Natalie has taken everything she can and left, and put out an APB.
They quickly find Natalie at a cheap hotel and she explains that Leonard had beat her for the last time and she packed and tried to leave. Marty Burgess, their downstairs neighbor, heard and got involved, and Mason came by and walked Natalie to her car. Hank finally tells Natalie that Leonard is dead, and she insists that Mason didn't kill him. They point out that she could have done it but Natalie says that she wasn't strong enough to kill her. When the officers wonder why she stayed with Leonard, Natalie admits that things used to be good and then she was afraid to leave.
Hank and Nick report to Renard, who sends them to talk to Marty at his junk shop. Marty, a meek little man, is seemingly surprised to learn that Leonard is dead. He admits that he was scared of Leonard, and that Mason got there after he did. As Marty talks about how mean Leonard was, Nick briefly sees his true face, a mouse-like creature. Marty tells them that his father can confirm he went back to his apartment. Next, the partners go to find Mason.
Juliette comes home and sees a man and woman parked outside in a pick-up truck, watching the house and taking photos. When she goes to talk to them, they drive off before she can get there but she gets a cell phone photo of their license plates.
The officers meet with Mason, a personal injury lawyer, who says that Leonard was an ass and they argued, but he didn't kill the man. When Mason takes a call and discusses a case, Nick sees him transform into a lizardlike monster. That night, he goes to the trailer and finds a description of Mason matching that of a lausenschlange, a monster that feeds on children. Nick meets with Monroe, who identifies Marty as a mausehertz and says that they're cowardly by nature. He also confirms that lausenschlange are extremely dangerous and prey on the mausehertz. Monroe gets a call about a promising job and Nick leaves.
At home, Juliette tells Nick about the two watchers and gives him the photos. He calls in the plates and assures Juliette that it's nothing serious, but then they hear something moving through the garbage cans. Nick goes out to investigate and discovers that a raccoon is responsible. Relieved, he goes back inside and gets a call from the station confirming that the pick-up truck belongs to John Oblinger. Juliette sees the address that he writes down.
At a garage, the killer meets with the owner, Andrew Salazar, who dares his visitor to take him to court. The visitor, sees him as a different, older man, stabs him in the chest with a tire iron, and then tosses the owner in the dumpster. The next day, one of the mechanics finds Andrew and calls in the police. No one liked Andrew and they figure that the owner's death is connected to Leonard's.
Monroe goes to the site of his new repair job, a seemingly-empty office. He gets no answer and several hooded creatures grab him and beat him unconscious.
Juliette goes to the address that Nick found and sees the woman from the truck and two children playing in the lawn. The mother sees Juliette and quickly gets her sons inside, and then looks nervously at Juliette from the window.
Marty comes to see Natalie and discovers that she's moving. Natalie thanks him for arriving just in time to save her and then gives Marty a hug. She then asks him to take all of Leonard's stuff to his shop and sell it for her, and then asks about his father. Marty says that he's about the same. Mason comes out with a box and Natalie explains that he's helping her move. The lawyer winks knowingly at Marty and takes the box downstairs. As Marty leaves, he approaches Mason's expensive car. Mason grabs him and says that Natalie is his and starts to choke Marty to death. A couple comes out and Mason is forced to release Marty, who runs to his car and drives off.
Monroe wakes up, alone, and goes back to his car, only to discover that his attackers have left a Grimm scythe drawn on the hood in Monroe's blood.
At the station, Nick discovers that Marty paid $1,200 to Salazar in car repairs and they decide to talk to the man's father. The landlord lets them in when there's no answer to their knock, and they discover that Marty is a hoarder. Looking around, the officers discover that Marty is burning incense to cover the smell of something worse, and find Marty's father, dead for at least two days. Nick figures that when his father died, Marty may have felt he was free at last.
Mason is at his office when Marty comes in and tells him to leave Natalie alone. The lawyer wonders if he wants to die and says that Marty doesn't ever tell him what to do because that's the way of the world. Marty steps forward for a moment but Mason advances on him and Marty cowers on the ground as the lawyer dismisses him as a loser. Marty sees Mason as his father, grabs a bookend, and savagely attacks him.
Natalie is walking home when Marty pulls up in Mason's Camaro and offers her flowers. She wonders where Mason is and Marty says that he couldn't make it, and claims that he bought Mason's car. He then invites Natalie to dinner and she accepts.
Monroe calls Nick as he goes over Marty's apartment and says that they need to talk, but Nick says that it will have to wait. Wu informs him that they've tracked down Marty's car outside of Mason's office. They find Mason, beaten to death, and put out an APB on the Camaro.
Marty bribes the head waiter into seating them at a fancy restaurant. They talk and Marty tells Natalie that she's smart and pretty and can do whatever she wants, and that he plans to move as well. She asks about his father and Marty says that he has to finally do things for himself. As Natalie admires his newfound self-confidence, he hears a father at the next table berating his son, telling him to grow up. Marty gets up and tells him to stop, and then hits him when he refuses to back down. Marty begins to see everyone in the restaurant as his father and Natalie quickly gets him out.
As Marty drives away, Natalie says that she admires Marty for what he did for the boy, and that her father was like that. Marty tells her that people like that deserve to die. He guns the engine and tells Natalie that he's been going too slow his entire life but he never will again. A police car pursues them and Marty manages to lose it in traffic and keep driving while Natalie begs him to stop.
Wu gets a report on the Camaro and tells Nick, who realizes that Marty is heading back to his shop, his safety zone.
Marty parks the car at the shop and pulls Natalie out. He tells her that they all want him to change back but he likes what he is and then hauls her inside. Nick and Hank pull up outside and find Natalie's discarded purse, and realize that Marty has Natalie with her. They go inside and search the place, calling on Marty to give up. Searching the shop, they finally locate Marty and tell him to release Natalie. Crying, Marty looks in the mirror and sees himself as his father. He yells that he isn't his father, releases Natalie, and crawls under a table and back into the maze of aisles. Nick goes after him and easily captures him, and tells him that they know Marty's father is dead. Sobbing, Marty says that he was dead but he's not, and that he's everywhere no matter how many times he kills him. When Nick says that he knows who Marty really is, Marty insists that nobody knows him.
When Nick comes home that night, he learns that Juliette went to see the woman. Juliette says that the woman was afraid of her and wonders why, but Nick doesn't have a good answer for her. She wishes she knew who the woman was but is comforted by the fact that she knows who Nick is.
Nick goes to see the injured Monroe, who explains that the people that attacked him were trying to send a message. He shows Nick a piece of paper with the scythe symbol on it, and warns him that they're not happy that Monroe is messing with the status quo. Nick says that he won't ask for his help anymore but Monroe refuses to run and tells Nick to ask him for all the help he needs. As he offers Nick a beer, Monroe promises that the next time they'll be ready for them.