Even though Brian Cassidy is a dedicated member of the Special Victims Unit, he lacks the language to describe sex crimes and the emotional maturity to deal with them. He views his partner,
John Munch (
Richard Belzer), as a mentor. Cassidy has a drunken
one-night stand with fellow SVU Detective
Olivia Benson (
Mariska Hargitay) but wants to pursue a relationship with her. She turns him down, citing a policy of not having relationships with co-workers. Years later, Benson expresses regret at the way she handled the situation. These factors and the stress of the unit wear Cassidy down. After Cassidy loses his temper at Benson in front of the squad, Captain
Donald Cragen (
Dann Florek) tests his readiness to deal with sex crimes by sending him to check up on a teenage rape survivor Cragen met when he worked in homicide. After she details how her abuser systematically violated her and then had her
gang-raped, Cassidy decides he lacks the stomach to deal with sex crimes and transfers to the
NYPD's Narcotics Division. Munch later laments that he felt abandoned when Cassidy became the latest in a long line of partners who left him. Besides a brief appearance whilst undercover as a bartender in
Season 9 episode "
Authority", Cassidy makes a proper return 12 years later in the
Season 13 episode "
Rhodium Nights". He has been working undercover for three years as a bodyguard for a
pimp, Bart Ganzel (
Peter Jacobson), and gives the SVU detectives information about Ganzel's prostitution ring. When a prostitute who had been dating both Cassidy and Ganzel is found dead in bed with Cragen, Detective
Nick Amaro (
Danny Pino) believes Cassidy played a role in framing Cragen, despite the other SVU detectives vouching for him. Amaro confronts Cassidy at gunpoint and tries to force information out of him. After Ganzel discovers Cassidy is working undercover to expose him, he has a corrupt NYPD police officer shoot Cassidy. Amaro helps uncover the conspiracy that led to the attack. Cassidy survives and awakens in the hospital, where Benson tells him she is not the same person she was when they first worked together, and kisses him. In the episode "Undercover Blue", a prostitute accuses Cassidy of raping her while he was undercover four years prior. Investigation reveals the woman and her boss set Cassidy up to make money from suing the NYPD, and the charges are dropped. Because of the longtime consensual relationship Cassidy had had with a prostitute during his undercover stint, however, he is demoted from detective to officer, working nights at a Bronx courthouse. Benson's and Cassidy's relationship, until now a secret to SVU, is also forced into the open in this episode when Amaro and Munch go to Cassidy's apartment and find Benson there; in
Season 15, the couple moves in together. In the episode "Internal Affairs",
Internal Affairs Bureau Lt. Ed Tucker (
Robert John Burke) puts Cassidy undercover to investigate a corrupt precinct with the promise that, if the operation is successful, he will earn back his detective badge. Although the assignment very nearly leads to his death at the hands of two corrupt cops, it is revealed in the episode "Rapist Anonymous" that Cassidy is a detective again and will be working in Internal Affairs. Cassidy and Benson amicably break up during the episode "Downloaded Child" after realizing they are fundamentally different people; he prefers to live in the moment, while she plans for the future. At the end of Season 19's "
Gone Fishin'", Cassidy returns as a
DA investigator and tells Benson he is investigating allegations that she is abusing her adopted son, Noah. In "Mood", it is revealed that Cassidy told another DA investigator about Benson and her ordeal with serial rapist William Lewis (
Pablo Schreiber), which indirectly led to the child abuse claim. Benson, furious, says she never wants to speak to him again. In "Chasing Demons", Cassidy testifies against a child molester he had physically assaulted during arrest. During
cross-examination, Cassidy explodes with anger and threatens to kill the defendant, causing a
mistrial. The suspect is murdered the following day after Cassidy is seen at his house, and ADA
Peter Stone (
Philip Winchester) has him arrested for murder. The killer, though, turns out to have been one of the suspect's victims, and Cassidy is exonerated. However, Cassidy opens up to Stone, telling him that he was molested as a child by his
Little League coach, something he never told Benson, and that one reason he left the unit was his inability to handle the recurring memories of his abuse. Cassidy later apologizes to Benson for unintentionally jeopardizing her custody of Noah, and reconciles with her. In "Facing Demons", the suicide of a 22-year-old man leads SVU to Gary Dolan (
William Sadler), who molested the victim years before. While searching Dolan's apartment, Benson recognizes Cassidy as a boy in one of Dolan's photographs of the many Little League teams he coached. Benson reaches out to Cassidy to see if he has information about Dolan. Cassidy claims to know nothing, but after the visit, Benson gives an update on the case to Stone, who realizes Dolan must be Cassidy's abuser. Stone wants him to testify against Dolan, but he refuses, partly to avoid facing the memories of his abuse and partly to keep Benson from finding out about it. He does help Stone recruit another of Dolan's victims to testify, but the man's testimony is
stricken from the record when he admits to drinking alcohol before testifying. Realizing Dolan will go free without any other survivors ready to testify against him, Cassidy, with Benson's encouragement and support, decides to tell the court what Dolan did to him. ==Development==