The owner of a bakery is shot by someone dressed up like a clown. When he leaves, witnesses tell the police where he went so they chase him and find dozens of clowns. Flack fills Mac on what they know that the victim is the owner named Cressida, that he was throwing a birthday party for his son, and that he hired a clown to entertain them. The clowns revealed that an advertisement on the web was offering $500 to whoever can best appear as the clown pictured.
Danny finds the costume the shooter wore. Meanwhile Mac tells Lindsay to work on the bullet. Danny finds some trace on the costume and in the shoes making him wonder if the man wore his shoes in the clown shoes. Adam tries to call the clown registry to find who owns the clown image that the shooter used. The man refuses unless he is given a warrant. Adam tells Mac, who proceeds to go there with Jo. When they get there the man is still belligerent but gives the name because Adam faxed the warrant. The man's name is Stan Ridgeway. Flack and Danny go to his apartment and find him tied and gagged. He tells them he was getting ready for the Cressida party when someone came in, knocked him and took his costume. He also left $200 which is what he was getting for the Cressida gig. Danny processes the scene and finds a fingerprint impression in the makeup. He sends it to Adam, who gets anidentification and shows it to Jo.
The print belongs to a guy named Bobby Reddon, who was arrested for a misdemeanor, which makes Jo wonder why would he kill Cressida. Adam also points out that 5 years ago he dropped off the face of the earth. Jo thenasks Macif the FBI called him. He says no and asks why. She tells him that Reddon went into the Federal witness protection program and that the Feds will come soon.They showed up and Jo tellsMac it's her ex-husband Russ Josephson. Josephson talks to Mac about Reddon, Mac tells him he's the prime suspect in the Cressida murder but Josephson claims Reddon is no killer. Mac thinks the FBI is trying to keep the fact that one of their witnesses is a killer. Josephson tells Mac that Reddon was placed in the program for the NYPD specifically on the request of Flack.
Mac talks to Flack about Reddon and he also says he's no killer. He goes on to say that Reddon was not some perp who ratted on someone to get out of a jam. He witnessed a murder and testified against the killers. Josephson goes to see Jo and clearly there's some animosity between them when he asks her out and she refuses. Lindsay comes in to tell Jo that the bullet that killed Reddon was used to kill someone in Memphis. Josephson says that's where Reddon was relocated. Later, they all come together and discuss what they know.The man killed in Memphis is from New York, and possibly a hitman sent to get Reddon.
ButReddon somehow got the upper hand and before killing got him to admit who sent him and they're thinking it's Cressida who somehow found out Bobby's in Memphis and sent the man. Mac says it's all good except for the fact that there's no link between Cresida and Reddon. Flack goes to see Reddon's girlfriend, Ainsley, who blames Flack for Bobby's inability to see their son. Flack tells her that Bobby left so he would not disrupt her life. Ainsley then refuses to take Flack's calls about Bobby. Flack leaves and thinks he sees Bobby so he follows him.
Hawkes tells Mac that both Cressida and the man killed in Memphis had traces of chemicals on their clothes that are used to make Meth,which is what the men who Bobby testified against were into. But still no link between them and Cressida. Danny learns the traces he found on the clown costume were from a crematorium. Jo later learns that Cressida is the godfather to one of the men Bobby tesified against. Danny finds the crematorium and tells Mac, who in turn tells him that no one has heard from Flack.
Flack goes into the crematorium and calls Bobby, who comes out with a gun aimed at Flack. That's when he tells him that he shouldn't have testified. Flack then tells him that sometimes that's how life is, but he tells him that if he puts the gun down he can still be a father to his son. But if he shoots him it's all over. He puts the gun down and tries to run but Flacksubdues him.Mac enters the crematorium. Flack tells Mac he shouldn't have made him tesify. Mac tells him that Reddon knows that what he did was the right thing. Before booking Reddon, Flack takes him to see Ainsley and their son.