After rescuing the failed inter-galactic mission sent out by the Council of Earth and Mars, Tardis brings Doctor Who and his friends to the planet Rhoos. They materialise on a weird plain that turns out to be the hand of the evil Cyclops, Fo. He has been sleeping through all this, but now wakes and departs, taking Tardis with him. As the Doctor and three of his companions (Hill, Shelley and Chertzog) fly around in their contra-gravity suits on a recce, they are captured by a mysterious spaceship.
It belongs to the natives of Rhoos, the Kaarks, who resemble humanoid chickens. The giant Fo has been terrorising them, attacking their cities and stealing their scientists. They have retreated into an underground base. After meeting with the Kaark Supremo, the Doctor agrees to lead a mission into Fo's lair. He is accompanied by the leader of the ship that rescued him, Ff'ni.
There, they find Tardis amongst a menagerie of animals Fo has collected. These he makes fight the captured Kaark scientists, as Hill notes, "just like the martyrs in the Colosseum". They flee to the ship, chased by a giant leopard.
Yet the Doctor cannot leave without freeing the Kaarks from Fo. Batting aside the leopard with his walking stick, he opens a black box. There is blue flash and a smell of ozone, and Fo falls down dead. The Doctor and his friends bid Ff'ni a quick good-bye; they are eager to return to Earth.
- Although the Kaarks say Rhoos is the eighth planet of the P'Tuu system, the Doctor states that it is "third of GO-star, eighty million miles from primary". It also has "less axial inclination than Terra".