Parts: 6
Writer: Terry Nation
Airdates: 7 April 1973 - 12 May 1973
Production Code: SSS
Creature Feature: Daleks, Spiridons
Story: After the Doctor's injury in a battle, Jo helps him into the TARDIS, where he sends a telepathic message to the Time Lords and collapses. While the Doctor retreats into a recuperative coma, Jo waits until the TARDIS lands on the planet Spiridon and then ventures outside in search of help. The landscape outside is less than pleasant - Jo finds herself struggling through jungle foliage and dodging the expectorations of various plants until she finally comes across a spacecraft.
The ship belongs to a group of Thals, who agree to go back out and find Jo's friend for her. They warn her to stay in the ship - the planet's invisible inhabitants are roaming around, and Jo would be safer to stay inside. As Jo hides from invisible intruders on the ship, the Thals manage to retrieve the Doctor. But before they can get back to the ship, an invisible creature approaches...but it is not a native of Spiridon. It is a Dalek.
Jo's encounter with the plant life of Spiridon has left her infected with a rapidly-spreading fungus, and soon she falls into the invisible hands of a native. Meanwhile, the Doctor and his Thal companions are captured by the Daleks. The Daleks have come to Spiridon to learn the secret of invisibility, and already they have had success in applying their new technology. And with it, they plan to unleash the greatest Dalek army ever assembled...and take over the universe.