The Benson Interruption
On the eve of his testimony against Bart Ganzell -- remember, he's the guy who framed Cragen (Dann Florek) for killing a hooker earlier this season -- Cassidy (Dean Winters) is accused of having raped Heather Riggs (Robyn Rikoon), then a prostitute, when he was undercover.
Heather and her degenerate-scumbag-gambler boyfriend Bobby Navarro (John Ventimiglia) cooked up the whole story, so that they could sue the city and collect a big payday that would settle his debts.
Heather's pimp sent her to Cassidy with instructions that he should "break [her] in"; he got out of having sex with her thusly: "I told her I was finishing a course of antibiotics, and she wouldn't want to catch what I had."
Amaro (Danny Pino), Munch (Richard Belzer), Rollins (Kelli Giddish), and Cassidy set up Navarro by tricking him into paying off his gambling debts to a drug dealer and getting him arrested, so that they can coerce him into confessing to the false rape accusation.
Benson (Mariska Hargitay), because this whole time her secret boyfriend was CASSIDY! WHAAAAAAAAAAAT? Also Amaro (see below).
They find some other dirtbags to corroborate Heather's story (the true one), and the D.A. drops the charges against Cassidy.
Apparently, even though he kind of thinks she never believed in his innocence, Cassidy and Benson are going to stay together. In other news, back when he was undercover a decade ago, before his marriage, Amaro had an affair with Cynthia (Andrea Navedo), a drug kingpin's sister, and they had a son together, who's now nine; until Cassidy's lawyer (Reg E. Cathey) found them in order to discredit Amaro's testimony against Cassidy, Cynthia never knew he was a cop, and Amaro never knew she'd had a child, but as the episode ends, father and son are having a super-awkward get-together.