Jo helps the Doctor into the TARDIS and he sends a message to the Time Lords via the telepathic circuits before collapsing. Jo helps him to a couch where he falls into a coma after telling her to record what happens in the log - a hand-held device. 

The TARDIS arrives on the planet Spiridon and Jo decides to go outside to find help. As she leaves the ship she is sprayed with a fungus liquid by some of the plants. She finds some footprints and follows them to a crashed spaceship in which there is a dead pilot (Alan Casley). She is found there by two Thals, Taron (Bernard Horsfall) and Vaber (Prentis Hancock). A third Thal, Codal (Tim Preece), calls the others away and Jo hides in a cupboard as an invisible creature explores the ship. 

The Doctor recovers in the TARDIS but finds that there is no oxygen. He is saved from suffocation by the Thals who remove the fungus growth covering the outside of the TARDIS and help the Doctor out. Taron sprays the Doctor to kill the fungus. If left untreated, it would cover his whole body. They go to find Jo, who has the fungus growth on her hand, but encounter an invisible Dalek. 

Jo is found and taken by one of the invisible Spiridons and Codal is captured by a group of other Spiridons. Two Daleks intend to destroy the ship, and the Doctor is captured when he tries to stop them, thinking Jo is inside. 

At the Dalek city, the Doctor is placed in a cell with Codal. Jo, however, is being treated by a Spiridon called Wester (Roy Skelton), who is against the Daleks (Murphy Grumbar, John Scott Martin, Cy Town; voices: Roy Skelton, Michael Wisher). 

Meanwhile Vaber wants to blow up the Dalek city but Taron advocates caution. Another ship from Skaro crashes. The only survivors are an old love of Taron's, Rebec (Jane How), Latep (Alan Tucker) and Marat (Hilary Minster). They have come to warn Taron that somewhere on the planet is an army of 10,000 Daleks. 

Jo recovers and gets into the Dalek city. The Doctor and Codal escape from the cell and Taron, Rebec and Marat try and gain entry by a network of icy passages. The planet is filled with an allotrope of ice which is liquid although below freezing. This substance can erupt from ice volcanoes, and an eruption starts as they are in the passages. 

The Doctor and Codal find themselves in the lower levels of the city and help Taron, Rebec and Marat out of a ventilation shaft before the volcano erupts and the Daleks close the ducts. Marat is exterminated as he tries to hold the Daleks off. The Daleks find a map on his body showing the location of the Thals' explosives and they go to destroy them, followed by Jo. The Doctor discovers that the Dalek army is frozen in suspended animation in a large cave. 

The Doctor and the Thals escape from the city by making a hot air balloon and allowing it to pull them up a ventilation shaft. They reach the top and push some rocks down the shaft to prevent a Dalek following them on an anti-gravitational disk. 

When the Daleks have set the timers on the Thals' explosives and have left, Jo manages to disarm two of the three bombs. 

The Daleks are preparing a bacteria which will destroy all life on Spiridon. Only the Daleks and their slave workers will be given immunity. 

The Doctor and the Thals meet up with Jo and they all decide to spend the night at the Plane of Stones, an area which retains the heat from the day. Overnight, Vaber leaves to try and destroy the Dalek base alone and he is caught by the Spiridons. Taron and Codal ambush the party as it returns to the Dalek city and takes their purple fur cloaks as disguises. Wester warns the Doctor and Jo that the Daleks are headed for the Plane of Stones and returns to the city to try and help there. Vaber is exterminated by the Daleks but Taron and Codal manage to get the bombs and return to the Doctor. They disable two Daleks by pushing them into ice pools and then one of the Daleks is used, with Rebec inside, to get the Doctor, Codal and Taron into the Dalek city. Latep and Jo are to get to the refrigeration unit down the ventilation shaft. 

In the Dalek city, Wester releases the bacteria early, trapping the immune Daleks in the isolation room forever and killing himself instantly. 

In the Dalek city, the Doctor's party is uncovered and they make their way to the lower levels, abandoning the Dalek casing along the way. Jo and Latep see the Dalek Supreme arrive to take charge of operations on the planet. They manage to get to the ventilation shaft and descend to meet the Doctor, Taran, Codal and Rebec. The Doctor initially plans to lock the refrigeration unit on, but the Dalek Supreme orders the Dalek army revived and the unit is switched off. The Doctor then decides to use Codal's bomb to open a fissure into the ice volcano and bury the Daleks in ice. 

Latep uses the second bomb to delay the party of Daleks that are heading for them. The Doctor sets the bomb in the fissure and it destroys the approaching Daleks as well as initiating an ice volcano which buries the Dalek army in ice. 

The Doctor bids farewell to the Thals. Latep, who has fallen in love with Jo, asks her to come back to Skaro with them on the Dalek Supreme's ship. She refuses. The Dalek Supreme chases the Doctor and Jo back to the TARDIS and they head back to Earth.

_{Synopsis from Doctor Who: The Third Doctor Handbook by David J. Howe, Mark Stammers and Stephen James Walker, reprinted with permission; further reproduction is not permitted. Available from Telos}