Cassidy (Dean Winters) and Olivia have a one-night stand in the very first season of the addictive series. Since they're both detectives at the same precinct, Olivia cuts it off because she doesn't want it to interfere with work — and that's the last we see of him, until years later.
We're reintroduced to Cassidy in the last episode of season 13 as he's working undercover for a prostitution ring. Olivia and Cassidy reconnect almost immediately, and season 14 is straight up chaos — Cassidy is shot and in life-threatening condition. Then, he's accused of rape and Captain Donald Cragen is accused of murder, which is all wrapped up in the same prostitution ring drama.
Ultimately, Cassidy and Cragen are let off the hook. Olivia and Cassidy continue their romance and even move in together! Sadly, at the end of season 15, the two grow apart and separate even though we know there will always be love between them.
