As the Brigadier meets with the Secret Vote Finance Committee in a vain attempt to convince them to keep funding UNIT, the Doctor investigates the mystery of five missing people. Each of the five people had the same address in their telephone books — a health clinic in a country manor house. The Doctor approaches the receptionist, asking for a meeting with Doctor Dantalion, but the receptionist only agrees when the Doctor mentions the amount he's willing to pay and the fact that he has no close relations.
Suspicious, the Doctor marks the receptionist's back with a pen. When Doctor Dantalion appears, the Doctor notes that she has a pen mark down her back — he is dealing with only one alien, who can change shapes. The Doctor confronts the alien, who admits to being a Birastrop and offers the Doctor a drink. However, the drink contains a paralysing agent, and Dantalion plans to cut the Doctor up as he did the other five missing people. The Doctor has a moment of regret that he told no one of his whereabouts, when Jo bursts into the room, holding a pistol.
Back at UNIT, while the Brigadier tries to contact various people and plead his case, the Doctor keeps interrupting him with the story of his adventure. Dantalion is being held in a cell and and interrogator is questioning him. The Doctor can't figure out why Dantalion was killing people.
In the laboratory, UNIT staff are drinking. The Brigadier enters and tells them that UNIT's funding has been cut off and their job given to the Royal Navy. The Brigadier was told that if they could find a cheaper location, they might be able to convince the government to reinstate their funding.
Jo takes food to the men guarding Dantalion and meets the interrogator as he is finishing up. She looks in the cell and notices that its casing is empty — then she yells for the guards, saying that the interrogator is Dantalion. Dantalion shoots the guards, and when Jo attacks him, she falls down the steps, hitting her head. Dantalion is recaptured.
When Jo wakes, the Doctor tells her that Dantalion saved her ilfe, at the expense of its own freedom. When he compares Dantalion's behaviour to a doctor, he realises that Dantalion is a doctor, and he's been capturing people for medical study.
The Brigadier is informed that his files are to be given to Rear Admiral Jonathan Zecca, who will take over UNIT's duties. Jo and the Doctor talk with Dantalion, who will soon be given into Zecca's custody. Dantalion tries to explain that cutting up people is how his species learns medicine. While Jo and the Doctor are appalled, he explains that his culture is just different from theirs.
A convoy takes Dantalion away. Meanwhile, the Doctor tries to communicate with the Time Lords, asking for a favour.
The truck carrying Dantalion experiences a brief moment of sluggish behaviour. Zecca decides to investigate, and they discover that Dantalion is no longer in the truck.
The Doctor bids farewell to Dantalion, having rescued him from the convoy. Before letting Dantalion leave, the Doctor gets him to sign over the country manor to him.
Now that UNIT has a new base of operations, the country manor, their funding is restored.
- Third Doctor
- Jo Grant
- Brigadier Lethbridge-Stewart
- Captain Mike Yates
- Sergeant Benton
- Corporal Bell
- Robert Walker
- Jonathan Zecca
- Ferrand, UNIT Interrogator from Geneva
- The Brigadier tries to talk to Sir Reginald Styles, and talks to Horatio Chinn and Jo's uncle.
- The Doctor tells Benton a story involving Medusoids.
- Dantalion is a Birastrop.
- Mike Yates mentions that a tissue sample was sent to Liz Shaw at Cambridge.
- The stories in Decalog 2 have a common theme of homes owned or acquired by the Doctor. The home in this story is the house he obtains from Dantalion.
- The Brigadier mentions the Nestene invasions, (TV: Spearhead from Space, Terror of the Autons) the attack on Wenley Moor (TV: Doctor Who and the Silurians) and the Sea Devils (TV: The Sea Devils) as examples of UNIT defending the Earth from alien attack.
- He mentions the First World Peace Conference, (TV: The Mind of Evil) the talks at Auderly House (TV: Day of the Daleks) and the disposal of the Thunderbolt missile (TV: The Mind of Evil) as examples of UNIT's other work.
- Secretary Walker first appeared in TV: The Sea Devils, though he was only given the first name Robert in PROSE: Doctor Who and the Sea-Devils.
- The Doctor cites the work he did for the Time Lords on Uxarieus, (TV: Colony in Space) Peladon, (TV: The Curse of Peladon) and Solos. (TV: The Mutants)