The Doctor has been wounded after being shot by the Master on the Ogron planet. Jo helps him into the TARDIS, where he uses the telepathic circuits to send a message to the Time Lords before he collapses into a coma. The TARDIS is being controlled by the Time-Lords and lands on a jungle planet. Some of the plants block the scanner with a thick sap. Jo leaves the ship and is sprayed herself.
The TARDIS is being covered by hardening plant sap. The Doctor awakes to find the oxygen supply deteriorating. In the jungle, Jo finds a spacecraft with a dead pilot. Three blond humanoids enter: Taron (Bernard Horsfall), Vaber (Prentis Hancock) and Codal (Tim Preece). They help, chipping the hardened sap from the TARDIS doors and dragging the Doctor out into the air. When they explain they are from the planet Skaro, the Doctor recognizes them as Thals, from the Dalek home world.
Jo is unconscious because of the plant sap and is removed from the spacecraft by an invisible humanoid called a Spiridon. On the way to the Thals’ ship the Doctor and the Thals find a disabled invisible Dalek. The Daleks are there to study the Spiridon to learn how to make themselves invisible, and to use them as slave-labour. The Daleks have already discovered the Thals’ spacecraft when the Doctor gets there. The Doctor, thinking that Jo is still inside, reveals himself, but the Daleks destroy the spacecraft anyway. There are some ten thousand Daleks on the planet, ready to be awakened.
The Doctor is taken to the Dalek base for interrogation and locked up with Codal. The sonic screwdriver will not unlock the cell-door, but Codal and the Doctor try to alter the TARDIS log to emit a frequency that will jam Dalek controls. Meanwhile, Jo is being cared for by the Spiridon who found her, whose name is Wester (Roy Skelton) and is trying to fight the Daleks. He cures her and tells her where the Doctor and Codal are being held. There is a lot of cliffhangers as everyone wanders about the Dalek base—Jo messing with dials as she hides from the Daleks, Thals trying to plant bombs, the Doctor hiding with the Thals, and when they are trapped, riding up an air-shaft in a jury-rigged hot-air balloon.
The Doctor and the Thals escape and flee the pursuing Daleks. In the cooling chamber, the Thals barricade the entrance while the Doctor sets a bomb to destroy the Daleks’ army, which is beginning to awaken. The Dalek Supreme has arrived by spaceship and exterminates the Section Leader for incompetence. Jo and Latep arrive at the cooling chamber just in time for the bomb to explode. Molten ice floods the chamber, freezing the Dalek army to death. The group escapes to the surface and makes its way to the Dalek Supreme’s spacecraft. The Thals leave for Skaro and the Doctor and Jo rush to the TARDIS and dematerialize just as the Daleks open fire on them.
The story received mixed reviews. The problem of the Spiridon being invisible is solved by making them wear big shaggy purple coats, as if Barney the Dinosaur became a pimp. At some point, some idiot at the BBC decided that the Dalek operators had to wear makeup in case someone saw them through the grill. The operators protested by wearing drag, and the voice-operators gave them camp voices, like the Monty Pythons. Apparently, there is no film of this, but I for one would pay good money to see it.