The TARDIS makes a return trip to the jungle planet of Spiridon, when the Doctor and Charley fall in with a lost platoon of shellshocked Thal soldiers - victims of a sneak attack by their blood enemies, the Daleks. Besiged by deadly flora, surrounded by invisible monsters, and with all hope of rescue gone, the Thals are the victims of a grim experiment in psychological warfare. With the very nature of reality under threat, the Doctor and Charley need each other more than ever. But dark forces are conspiring to tear them apart...