Synopsis: The TARDIS is forced to land on an alien world infested by plants and fungi. The Ugrakks have learnt of the Doctor and his TARDIS from an Earth space traveller, Professor Lammers. Under constant attack from the giant Zama flies, the Ugrakks are desperate to escape their world to find a new planet. Unable to travel in space by conventional means, the Ugrakks are eager to make use of the Doctor and his TARDIS which will be able to transport them without ill effect. Unwilling to submit to the demands of the Ugrakks, the Doctor takes advantage of a Zama fly attack and escapes with Professor Lammers. When the Doctor discovers the Zama fly grubs beneath the organic city of the Ugrakks, he hatches a plan that will put an end to the evil plant creatures once and for all.