Sponge (The Sinister Sponge)

A juvenile sponge from Femizor escaped its pen and played with a molecular disperser, which sent it in orbit around Inscruta. Entry into the atmosphere contaminated the sponge with Oriolic dust beams, which disintegrated its male hormones. As sponges have an even balance of male and female hormones, this made half of the sponge feel like dying and the other half feel lonely. As its sickness started affecting the male Instrutes, the sponge began telepathically dominating the females, lessening their sensitivity to sound and turning them against the males.

When the Fourth Doctor arrived on the planet with his companions, the sponge quickly captured Sarah Jane Smith. However, when confronted with a Rhoa, the sponge agreed to free the women; the Doctor then created an elixir to cure the males' affliction and used a Molecular Speed-beam to return the sponge to its home system. (PROSE: The Sinister Sponge)

The robot dog read to Fenella Frimbly about their Master's visit to a world where the women had fallen under the influence of a yellowish-green cloud of vapor. Fenella recognised the monster as the "Sinisterest Sponge". The Master had brought it to Baker's End in his space machine on the way back to "its home dimension, somewhere or other," but forgot and left it in the downstairs bathroom. It hung there in the shower in a bad mood, and Fenella had to clean up after it, even though it tried to take over her mind. (PROSE: Mrs Frimbly's Festive Diary)