Injured by the Orgrons, the Doctor asks the Time Lords to send the TARDIS after the Daleks. Leaving him to sleep and recuperate, Jo goes to explore the planet Spirodon.
The Doctor recovers from his coma and befriends a Thal expedition but is captured by the Daleks and taken to their base. Then a second Thal expedition arrives with terrible news...
In the Dalek city, the Doctor breaks out of his prison cell and joins forces with the Thals. With the Daleks in pursuit, he is forced to improvise a daring escape plan.
The Doctor and the Thals escape from the Dalek city and the Doctor is delighted to find Jo is alive. But the Daleks are preparing deadly bacteria to kill all life on the planet.
Desperate to prevent the Daleks from releasing their plague, the Doctor, Jo and the Thals infiltrate their base. But the Daleks detect the intruders!
A force of ten thousand Daleks is coming to life below the surface of Spiridon. The Doctor and the Thals must find a way to defeat the Daleks before they invade the galaxy.