Season 4, Episode 7:

A Syrian diplomat indicates he would like to begin secret talks with the UK. Fiona volunteers to go undercover and liase with the diplomat but Adam's concerned: Years ago Fiona was locked in to a disastrous marriage with a Syrian secret service officer, Farook, whom Adam saved her from by framing Farook, resulting in his execution. Ever since Fiona has lived under an alias to avoid detection by the Syrians. Checks are made and since no one from her past is present Fiona is allowed to make contact. As the team prepare to work covertly with the diplomat it becomes clear that Fiona has another agenda, one she is arming herself for: Fiona has discovered that her first husband is still alive and looking for her. Fiona wants to provoke a confrontation so that she can kill her ex-husband and finally escape her past. However she fails and is kidnapped by Farook. Adam must save Fiona before she is flown out of the country. They nearly make their escape when Fiona is shot. She dies in Adam's arms.