Professor T’s work as a consultant for the police has begun eating into his professional obligations. You had a fish act as your substitute for a lecture, the dean complains to him, before rescinding any complaints—detectives have walked in, and he doesn’t want to criticize them.
A six-year-old child, Charlie Turner, has gone missing. She was home alone with her father, Max, when her au pair, Sally—who was off that evening—came back to find Max beaten unconscious and Charlie gone. Charlie’s mother, Lucy, returned just after that. All Max remembers is a security light going on, opening the door, and then being jumped.
Professor T is brutally blunt in his interactions with the distressed Turners. Kidnappings typically involve close family members, and while he doesn’t believe Lucy was involved, he suspects that Max knew his assailant and some score-settling was involved. Max vehemently denies it.
But Lucy suspects something, too. She asks Max, alone, if Kevin could be involved. He couldn’t be, Max insists. Lucy asks Sally to stay elsewhere for a few days, to give them some space.
Then the Turners receive a ransom note asking for 100,000 pounds in cash, accompanied by a lock of hair. Lucy lashes out at Max, accusing him of not believing Lucy is worth it. To confirm that the hair is Charlie’s, Lisa and Dan ask for a DNA sample from Lucy—but Charlie is adopted, as Professor T had already surmised. What about a sample of hair from a stuffed animal Charlie holds often? But her beloved monkey, Mops, is also gone, Lucy realizes. A toothbrush suffices.
As Lisa and Dan work through the night on the case, Lisa snaps at Dan. Stop being so nice to me! she says, before kissing him. She cuts things off quickly, however: she can’t do this while Charlie is still missing.
Max does have a record: he and Kevin, his business partner, were accused of importing mobile phones and avoiding taxes, but the case was dismissed. When Lisa and Dan visit Kevin at his factory, he says that they had used a dodgy intermediary and reported themselves; it was nothing nefarious.
The Turners receive an old cellphone in the mail with a message on it instructing them where to leave the ransom for Charlie. Max and Lucy decide to accede to the demands, and leave the cash at the drop-off.
Lisa has a breakthrough: looking up Sally Bains, the au pair, she discovers that she has been dead for two years. The au pair’s real name is not hard to discover, given that she is still friends with Sally on Facebook. Sally was an older friend, Nicki Trent explains under questioning. She gave Nicki an old ID for alcohol and let her stay at her house after Nicki’s mother kicked her out. Nicki served some time in prison for burglary, during which time Sally overdosed. Nicki needed a job when she got out, and renewed Sally’s old ID to assume her identity in order to more easily find work.
When accused of being part of the kidnapping, Sally instead says that Charlie can be a demanding child, and that Max isn’t patient with her tantrums or her bedwetting. After a weekend off, Nicki returned to find Charlie bruised; Max said she had fallen down the stairs. Security footage confirms Nicki’s alibi at the time that Max was attacked and Charlie disappeared.
Accused of abusing his daughter, Max denies it but admits that he did know his assailant—he was sent by Kevin. Four hundred thousand of Kevin’s pounds had been stolen while in the care of Max, who had been tired and left the money in a car while staying in a hotel. Kevin believed Max had stolen the money himself. Max also admits that he and Lucy paid the ransom, but haven’t heard anything since.
Professor T doesn’t believe Kevin was involved in the kidnapping, even when Mops is found in his warehouse during a search. Rabbit objects to Professor T and his methods. Lisa tells Rabbit not to let his emotions color his judgment; he snaps that he won’t take lectures on professionalism from her while she and Dan have their romance going on. Professor T jumps in: clearly Rabbit has been using alcohol as an anesthetic for the loss of his daughter. Rabbit attacks Professor T. Christina puts Rabbit on sick leave.
Nicki visits her mom and asks, did you ask for money? This wasn’t about money. Her mom responds that Nicki is the one who asked for her help. The ransom is still there, her mom says. When they go to retrieve it, however, they are surrounded by the police. Nicki’s mom threatens Charlie with a knife but is eventually talked down. Professor T had told Lisa to put the money back after the police had retrieved it—he figured everything out after visiting the Turner home and noticing a photo of Charlie missing from an album.
It’s the broken windows theory: one crime incites more crimes. Nicki found Max beaten on the floor of the Turner home and decided to kidnap Charlie to save her from what she thought, wrongly, was an abusive father. She put Charlie in her car, then called her mother to take her when Lucy returned home earlier than expected. Her mom is the one who asked for a ransom and planted Mops at Kevin’s factory.
A further wrinkle: Nicki is Charlie’s biological mother. She has long regretted giving Charlie up for adoption. As Nicki is taken away, she apologizes to the Turners, and gets a tight hug from Charlie.
Professor T’s relationship with his mother isn’t as complicated as that, but it is difficult. After much avoidance, he attends her art exhibition—and leaves immediately. He does buy a number of her paintings (all of her dog) anonymously, and locks them in the attic.