The Doctor and Sarah had been visiting the planet Yula when the Doctor sensed a disturbance in the Communal Will, the shared minds of the planet's inhabitants. Investigating, he traced the source to high levels of psychic energy coming from Crellium, a supposedly uninhabited swamp planet.
Landing on Crellium, the Doctor and Sarah detect the distant sound of someone sobbing. They follow it to a clearing filled with rows of flowers where a woman with no face is leaning over the body of an old man. The woman, Marsalla, telepathically explains to the Doctor and Sarah that Krem-ling had told her that they would be coming to Crellium from Yula, but not why. Krem-ling is the dead man, whom she has served since she was a child, and continues to serve in death by preparing for the time he will live again. Marsalla reveals that Krem-ling has taken on many forms before his current one, and now intends to take on the form of the Yulians. The Doctor realises that Krem-ling is from Drakka, a psychic vampire which feeds off of and possesses his victims. Normally only able to occupy one body at a time, by taking over the Communal Will of the Yulians he will be able to feed off entire worlds at once. He is already transmitting their psychic energy through the flowers into the emissaries of Drakka, who rise from the ground.
The Doctor shouts at Sarah to run back to the TARDIS, while he tries to convince Marsalla to help him stop Krem-ling. If she enters Krem-ling's body before he returns from Yula, he will die and the emissaries will follow. Marsalla protests that she would die as well, but the Doctor responds that a life of serving evil is no life at all, and he urges her to remember her life before Krem-ling: her home, her childhood, her friends and family. Convinced, Marsalla enters Krem-ling's body, and while the Doctor escapes, she activates a device which sets off a massive explosion, destroying herself and the vampires.
Back on Yula, the Doctor detects a difference in the Communal Will, a sense of greater activity, that perhaps they were becoming less complacent; and furthermore, the spirit of Marsalla has somehow found rest there.
- The navigational booster is a component of the TARDIS.
- The Yulians of Yula are a race of snailmen which possess a Communal Will.
- Sarah is reminded of the sirens of mythology.
- Marsalla is from Juksta, the natives of which have yellow skin and lack faces.
- Krem-ling is from Drakka, the natives of which are psychic vampires. He had previously taken on the forms of the Heemies, Worgs, Benlithulans and Suasion Norbs.
- to be added