Jo is working for Professor Mirko Vetich and he has created a device of great interest to the Master.
Professor Mirko Vetich has created an ionic vibrator. However, despite numerous tests alongside his assistant Jo Grant, it does not work. From afar, the Master watches his work. Mrs Prentice, one of the cleaners at UNIT HQ, is working near the device when she falls and hits her head against it. When she regains her senses, she hears a voice in her head, the voice of Zex, a being from the gger system on the fourth paraleptic level of the galaxy. The Master uses his own ionic vibrator to contract Zex through hyperspace and convince him to help conquer Earth.
Zex possesses Mrs Prentice and has her attack the soldiers at the Headquarters. She quickly overwhelms Mike Yates, the Brigadier and the soldiers. The Third Doctor and Mirko build an astro modulator, a device that upsets the equilibrium of beings from the fourth paraleptic level. After it is used against Zex, he is banished and Mrs Prentice awakens. The Doctor tells her she banged her head, but will be fine now.
- While possessing Mrs Prentice, Zex uses her mop as a weapon, throwing out force fields to hurl soldiers about.
- This story refers to the Doctor as "Doctor Who".