The episode begins with a flashback. A man pulls up beside a boy and offers him a ride. When asked why he has balloons in his car, the man says those are for the boy's parents. Back in present day, Sherlock and Watson are investigating the latest victim of the kidnapper. Outside of their home he notices torn branches, and notes that the victims are usually chloroformed- this victim was awake and fighting her abductor. When he goes inside he finds the parents begging to the media to have there child back, and Sherlock spray paints the camera lens explaining that the victims have less of a chance of survival the more the parents are on TV.
Sherlock asks the parents where they were when their daughter was taken, and the father claims he was buying wine. When Sherlock looks in the fridge he finds a bottle of wine which turns out to be different from the wine the father claimed to be buying. It's then that the father admits he was talking to a woman that he dated while he and his wife were separated. Sherlock works out that an unrelated police siren spooked the kidnapper into getting away without chloroforming the victim properly, and this woman is a valuable witness.
Tropes:
- A Match Made in Stockholm: Subverted with Adam. When he's found he makes excuses for Abbott and acts like someone who's come to sympathise with his captor. Sherlock eventually works out that Adam's faking it to make himself look sympathetic. In reality he didn't care about Abbott at all, only seeing him as a tool to acquire new victims.
- Alone with the Psycho: Holmes confronts Adam Kemper in his bedroom and explains to him how he deduced he was manipulating Samuel Abbott and not the other way around. He later meets him in the park to tell him how he found a hole in his immunity deal.
- Asshole Victim: The first kidnapped kid is the one who is torturing and killing other children for sadistic pleasure.
- Calling Card: The Balloon Man leaves behind balloons after abducting a child.
- Dartboard of Hate: After helping get the supposed accomplice of a Serial Killer an immunity deal before realizing he is the killer and not the accomplice, Sherlock tacks a copy of the immunity deal to his wall and started throwing knives at it.
- The Dog Was the Mastermind: While Adam Kemper was abducted, he used his greater intelligence to manipulate his abductor into a submissive relationship and then had him continue abducting children just to see the parents squirm at press conferences.
- Double-Meaning Title: Holmes was after a serial killer who targeted children. The killer is a child who is a predator.
- Driven to Suicide: Samuel Abbott after being arrested.
- Missing Child: Children are mysteriously kidnapped and their bodies are later found. The latest victim was taken from her very bedroom.
- My God, What Have I Done?: Sherlock goes through this when he realized he helped the true mastermind walk out of the police station with a full immunity agreement. Sherlock makes up for this by using the agreement's Exact Words to make sure the killer is punished anyway.
- Rules Lawyer: Ultimately how Sherlock manages to use the agreement against Adam; the immunity deal only covers abductions he committed with Abbott, but for one abduction Abbott was in hospital for serious back surgery, so Adam can be convicted for that one as he did it on his own.
- Serial Killer: Samuel Abbott AKA "The Balloon Man" who kidnapped children, leaving a batch of balloons on the site of the disappearance, who would turn up dead or never be seen again. It turns out his first victim Adam Kemper had mentally overpowered Samuel and was the one doing the killings.
- Shirtless Scene: We first see Sherlock sitting with no shirt on and looking through some boxes. The funny part is he remembers having it on when he started his search the previous night and has no idea how or why he took it off.
- Teens Are Monsters: Adam Kemper, the so-called victim of kidnapper Samuel Abbott. He actually abused his kidnapper and forced him to kidnap and kill children, then pretends to be a guilt-ridden victim of Stockholm Syndrome to get an immunity deal for his crimes. Thankfully Sherlock pulls a Rules Lawyer and get him convicted.
