Based on the real Paul Bernardo-Karla Homolka crimes. Lynley is back at work and busy leading the investigation for missing high-schoolgirl Sarah Middleton. Sarah was last seen talking to a red-haired woman in her mid-20s. One week after her disappearance, Sarah's body is dragged out from a pond. It appears that she was raped and then slit her wrists to escape her captor.
Soon afterwards, another high-schoolgirl, Kelly Stevens, goes missing. The possibility of the two disappearances being unrelated is ruled out when a witness reveals that she saw Kelly showing directions on a map to a man and a woman with red hair. The map in question is found at the site, and fingerprinting reveals two sets of fingerprints, which, however, have no match in the police database.
As the police begin sending appeals to the mystery red-haired woman to come clean and turn in her partner, a red-haired woman does turn up at a hospital, seeking medical assistance for a black eye and cut lip. The woman is Tania Thompson, and on Havers' questioning, she reveals that she was beaten up by her husband, Guy. Havers illegally collects Tania's fingerprints, and Lynley attempt to track down Guy and detain him on the pretext of domestic abuse.
However, neither Tania nor Guy are easy to crack, and time is running out, since Kelly is diabetic and cannot survive very long without her medication. Moreover, Lynley and Havers have different opinions on how to run the case, and risk a major falling-out.
- Abuse Mistake: Lynley is convinced that Havers is being carried away by her sympathy for Tania to think of her as Guy's victim instead of his partner in crime. He's right.
- Alone with the Psycho: Subverted. Twice.
- Beauty Is Bad: Guy Thompson.note As DC Harvey remarks, it "doesn't look like he needs to abduct women." Also, Tania.
- Big Bad Duumvirate: Tania and Guy.
- Character Development: Once upon a time, it would have been unthinkable for Lynley to shout at a woman, possibly because of a view that it is ungentlemanly to show anger to a weaker party. Over the years, his marriage to Helen and friendship with Havers have taught him a few things...
- Domestic Abuse: The reason why Tania comes to Lynley and Havers' notice. Subverted — Tania injured herself to emotionally blackmail Guy into committing the second kidnap.
- Evil Costume Switch : For most of the episode Tania is shown wearing blue jeans and a flowing jumper with a fringed scarf-like neckline, which makes her look soft and fragile. After Lynley and Havers work out that Tania is actually Guy's partner in every sense, including rape and murder, and are looking for the video tapes to prove it, Tania switches into tight black jeans with a matching fitted black leather jacket, with a distinct Evil Is Sexy vibe.
- Females Are More Innocent: Discussed — or, more accurately, "yelled about" — during Lynley and Havers' argument about Tania:Lynley: She trapped those girls knowing what Guy had planned!Havers: He had complete control over her, physically and mentally. You don't understand that fear and isolation...Lynley: Because I'm a man?Havers: Maybe!Lynley: Oh, and she's a poor helpless victim with no will of her own? Isn't that just a little bit sexist?
- Feud Episode: Lynley and Havers repeatedly anger each other with their conflicting takes on the case, and even have a shouting match at one point.
- Gut Feeling: Barbara's uncanny knack for sniffing out true villains completely fails her in this episode.
- Ironic Echo: All his talk of finding out one's enemy's "weak spots" and "weaknesses" comes back to bite Guy once Lynley becomes his enemy.Lynley: I found your weakness.
- Jerk with a Heart of Jerk: Guy. Yes, he had a Disappeared Dad and a an embarrassing mother. But his response to finding out that his dad had another family was to beat up his mother.
- Love Martyr: Tania persists on staying in her marriage with Guy despite the fact that he is a domestic abuser, emotional blackmailer and forces her to help with his crimes. Subverted, since Tania's is no martyr. He's never physically hurt her, and she's an equally keen participant in his atrocities.
- Masochism Tango: Guy emotionally blackmails Tania into compliance with his plans by making her repeat her marriage vows "to stand by her man." On her part, Tania calls Guy her "husband and master". In a nutshell: he's a sadist and control-freak. She's a sadist and Yandere. Both are perverted sociopaths.
- Mysterious Past: One line of Barbara's — namely, "...you don't understand the fear and isolation..." in her shouting match with Lynley — hints that Barbara may have been subject to some kind of domestic abuse before she met Lynley. This is supported by an earlier line from "Word of God" that she's "never had anything [she] could call a 'real relationship'", and certainly her behavior at the start of the series could be a result of that kind of abuse. But this is never explicitly stated in the text, and as this abruptly became the last episode, it was never explored further.
- OOC: Lynley's "WHERE IS SHE?" to Tania. Thankfully, Havers takes him aside for a word before it gets worse.
- Parental Issues: These provide Guy's Freudian Excuse for his sadism.
- Psychotic Smirk: Tania's expression at the end.
- Sadist: Guy. According to an ex-girlfriend, he once said that the two most erotic words in the dictionary are "stop" and "no". He met his match in Tania.
- Screw the Rules, I'm Doing What's Right!: The only reason the investigation proceeds at all is that Lynley and Havers don't agonise too much over illegally collected evidence and smokescreen charges.
- Spotlight-Stealing Squad: Honeysuckle Weeks and James D'Arcy, playing a husband-and-wife Big Bad Duumvirate, very nearly succeed in diverting fandom's focus on the Fan-Preferred Couple.
- Title Drop: A rare visual example.
- Unholy Matrimony: Honestly, just how did a sadist, twisted human being like Guy manage to find for a mate the equally sadist, twisted and Yandere Tania? What's worse, it happened in Real Life.
- "Well Done, Son!" Guy: Guy's father, Mike. The one thing that seriously unsettles Guy is the possibility of his father's disapproval. Lynley uses it to great effect.
- The Worf Effect: A subtle instance when Guy is introduced. Not many people have the personality to overshadow Lynley's, but Guy comes across as the more charismatic and level-headed of the two when they first meet at the seminar. That impression doesn't last for long once Lynley finds out Guy's "weak spot".