Fo was a giant Cyclops from the Black Galaxy. He had a hair-covered body, paw-like hands, tusk-like teeth, a snout-like nose, and a single eye in the centre of his forehead. Fo, an outcast from his race, was set adrift in space, but eventually he crashed on Rhoos. He began ravaging the planet, killing many Kaarks and capturing others to make them his playthings. Some of his victims were scientists whom he forced to construct defences for his stronghold.
When the First Doctor and his companions arrived on Rhoos, the TARDIS materialised on the sleeping giant's arm. Shortly after discovering this, the Doctor and three of his companions were captured by Kaarks led by Ff'ni. When they were returned to Fo's location, they found that the giant had awakened and left, taking the TARDIS with him. After learning about Fo from the Kaark Supremo, the Doctor and his companions were taken to Fo's stronghold. Fo captured them and put them in his model town, but the Doctor made his way to the TARDIS and got a black box from the storeroom. After saving Ff'ni from a Rhoos leopard, he placed the box on the ground and opened the box, which somehow killed Fo. (PROSE: The Playthings of Fo)