Susan Gantry calls Nash for help - she is being stalked by gang member Chili Gun. Chili Gun has raped Susan twice before. Chili Gun chases her to her car, but she drops the keys. A mysterious man in a hooded mask shoots Chili Gun dead, as the SIU and patrol car arrive. The masked man uses an inhaler before escaping. The masked man has saved Susan.
Serial rapist Chili Gun is a member of the 12th Street gang. Rafael Watson is head of the gang. His brother Tyrell has just been released from prison. Rafael has an alibi - he was at his chicken take-out restaurant all evening. Inhaler residue left at the crime scene reveals that the masked man suffers from asthma.
The masked man calls Nash proclaiming he is Nash's new partner. The masked man is a vigilante seeking justice. His cell phone call is traced to Raymond Gantry (Susan's ex-husband). But Raymond is an alcoholic, and Nash is sure Raymond is not the shooter. Later, the masked man calls again, and the call is again traced. The call is from Rafael's restaurant, where Rafael is found dead with a syringe sticking in his neck. The SIU begins a round up of all the members of the 12th Street gang.
Meanwhile Hagen Bridges visits Nick. At the SIU, Hagen sets fire to his jacket after putting his lit pipe into the jacket pocket. Hagen is a Constable with the Glasgow PD. Hagen has a dual mission. He seeks a Haggis dirk, a knife which Nick owns. The dirk was given to Nick by his great Aunt Pamela, and Nick now uses the knife to scale fish. Hagen claims that the dirk, once authenticated, will prove their roots are linked, that Hagen and the Bridges share a family heritage.
Hagen is also after Hans Kindler, alias Franz Planc, an international art thief who has stolen paintings from the Edinborough Museum. Hagen and Nick visit the Planc Gallery and confront Franz Planc. Hagen shows off the dirk as bait, but he accidentally drops the dirk and stabs himself in the foot. Planc bites - he sends his thug Tor to follow Hagen and steal the dirk.
Three members of the 12th Street gang are interrogated. Their lawyer refuses SIU help to figure out who is trying to eliminate members of the gang. The masked man calls Nash. The masked man is outside the SIU, waiting for the gang to exit the SIU. As the gang members exit, he guns down all three gang members, including their lawyer, with a silenced sniper rifle as Nash watches with outrage.
Hagen realizes they (he and Nick) are being followed. But Hagen mistakenly thinks their stalker is a young woman. Hagen attacks her, but she defends herself vigorously, forcing Hagen to use the dirk. Tor intervenes and grabs the dirk, but not before Nick punches Tor in the face. Tor escapes with the dirk. Back at the SIU, Hagen miraculously picks out Tor Thornston in the very first mug book he looks at.
All the 12th Street gang members have the same Parole Officer (PO). Antwon and Cassidy visit the PO, but he is new. The former PO to Rafael Watson and Chili Gun - Edward Bender, quit. Bender also has asthma. The SIU raids Bender's apartment and find a shrine to his murdered wife. She was killed in a bomb blast - the 12th Street gang was suspected. Also in Bender's apartment is bomb making materials. This time, Nash calls Bender, telling him to watch out.
Bender, pretending to be a PO, arrests Tyrell Watson at his halfway house. Bender handcuffs Tyrell to a similar booby-trapped car in which his wife Rose was blown up in. But before Bender can force Tyrell to drive off, Nash is there to arrest Bender.
Hagen, in disguise as a cowboy, confront Planc and Tor at the Gallery. Tor attacks Hagen with the dirk. Nick stands outside, unaware of what is going on inside until Hagen comes crashing through the front window. Antwon intercepts an escaping Planc and Tor in the back alley. Planc drops a porcelein statue containing stolen paintings.
Meanwhile, Harvey has gotten romantic with Janice, his neighbor and dog walker. Janice's dog Ness dislikes Harvey. Harvey puts the dog outside the bedroom, but the dog viciously bites Harvey in the groin.
Nash and Nick decide to donate the Haggis dirk to the Edinburg Historical Society so that the dirk can be authenticated. Hagen takes the dirk with him and says good-bye to Nash and Nick, but not before getting his coat caught in the elevator at Nash's apartment.
For more, see commentary for Episode 116