When the body of a man named Aiden O'Connell turns up shot and burned in a sugarcane field on Maui, Five-0 is called in to investigate. Phone records point them in the direction of Aiden's therapist, a powerfully seductive woman named Olivia Victor. From the moment he first meets her, McGarrett is convinced that she's responsible for Aiden's murder, but she was smart and clever enough to eliminate any evidence tying her to the murder. Olivia denies it but seems to get a sick pleasure out of teasing McGarrett, reaffirming his hunch that she's the killer. His obsession with her leads Olivia to file for a restraining order against him, throwing another roadblock in his path. Eventually the team uncovers payments Aiden made to a private investigtor, but when they go to visit the PI, they find that he too has been killed. Kono and Fong work to decrypt computer files the PI had on Aiden and seven other of Olivia's 'patients' and eventually learn that Olivia is actually a madam, and not a PI. She makes an effort to flee the islands via an international flight, but McGarrett and Five-0 manage to corner her and take her down on the plane as it taxis down the runway.
Meanwhile, Catherine teams up with disgraced WITSEC Agent Chris Channing, who she manipulated in episode #302 to gain access to intel on McGarrett's mother, Doris. Channing has intel that one of Doris' old targets, a Columbian assassin named Mangosta, actually faked his death and is out for revenge, meaning that Doris is once again in danger on the island. Channing and Catherine try to corner him at a hotel, but he eludes them and goes after Doris. Catherine races to save her, but instead finds Doris torturing Mangosta for information. In the end, Doris is safe, but Catherine has seen a very violent side of her that she did not know existed.