The TARDIS lands in a forbidding jungle, but with the Doctor slipping into a coma, Jo sets out to see if she can find help. She comes across a party of Thals, and stays in hiding aboard their crashed spaceship while they go to help the Doctor. After they have treated him, they tell him that they have travelled to this planet, Spiridon, to destroy a group of Daleks who have come to try and uncover the secrets of invisibility discovered by the natives.