True Believers (1) - S6.E12

When Nicco investigates the death of a female tree planter that took place thirty years ago, his best suspect is a well-known humanitarian Ian Northby, and Nicco soon begins to regret getting involved in the case, and finally realises that he doesn't belong in the Squad, but back on the force. After having an affair for months, Len gets antsy when Sonia gets involved in an undercover prostitution bust. She promises to quit if he will leave Wanda for her. He finally does, just as Sonja is being injected with a massive dose of heroin by the American pimp she's investigating. At the urging of the daughter of convicted murderer Abdul Rhaman, Ali agrees to look into the crime he was convicted of, the murder of Khallid Fulani. When she starts searching, she learns that Fulani never truly existed, and C.S.I.S. has seized all the files.