No sooner than Doctor Who, Sarah, and Harry arrive on Inscruta does a small green-yellow cloud envelop Sarah and carry her away struggling. The Doctor and Harry give chase, but they're ensnared by chameleon-tongue-like tentacles and dragged into the huge red petals of a man-eating flower. They narrowly escape the acid by singing and shouting, as life on Inscruta is unaccustomed to sound.
Just before nightfall, the Doctor and Harry find Elkalor, an Inscrute leader and friend of the Doctor. He helps them inside his home, a large cabbage, and tells them what has happened on Inscruta since the Doctor's last visit. All the males were stricken with a disease which caused their feathers to fall out, their skin to turn transparent, and their sensitivity to sound to worsen. At the same time, female Inscrutes became less sensitive to sound, and they began holding mass meetings in council hall where they would question their males' judgment and leadership with great shrillness and volume. The males discovered that the women had been hiding, and telepathically communicating with, a giant sponge in the council hall; when they objected, the women drove them from the city with ceaseless shouting and chattering.
Elkalor rejects the Doctor's offer of help, as his people's tradition forbids him from accepting aid; however, when Harry begins to turn transparent, he resigns himself to the Doctor's interference. That morning, Doctor Who fetches equipment from the TARDIS, and Harry makes himself earmuffs. All the men gather in the centre of the cabbage patch to face their wives and daughters, who come banging pots and pans, accompanied by the sponge. Sarah is among them, her face contorted into a snarl. But the Doctor calls out in a throaty garbling noise, and he and the sponge have a conversation with Femizorian aurapathy. When the sponge refuses to tell the truth, the Doctor confronts him with a Rhoa, the enemy of sponges throughout the cosmos.
The sponge confesses that it is an escapee from Femizor who arrived on Inscruta after playing with a molecular disperser. Passing through Oriolic dust beams affected the sponge's male hormones, an affliction it passed to the male Inscrutes; the sponge then telepathically controlled the female Inscrutes to do its bidding. The Doctor agreed to teach the Inscrutes the recipe for the elixir which would cure the hormonal affliction, and the sponge released its telepathic control; the Doctor then returned the sponge to Femizor.
With Sarah freed and Harry and the Inscrutes cured, the Doctor goes to relax at the Lake of Sighs.
- Femizor is a gigantic sponge colony in Alpha Mardis 2.
- Sponges are born with both male and female hormones and must be kept inside pens until maturity to control their reflexes and thought patterns.
- Fishing is said to be excellent at the Lake of Sighs.
- The Fourth Doctor cannot remember the Galactic Federation Sign Language.
- The Ergap is a Inscrute unit of time.
- The Spectrons of Spectro have seven ears.
- The sponge was sent to Iscruta by a molecular disperser. (TV: Doctor Who and the Silurians)
- The robot dog read to Fenella Frimbly about their Master's adventure with the "Sinisterest Sponge". (PROSE: Mrs Frimbly's Festive Diary)