UNIT has been seconded to handle security at Breathe Industries’ revolutionary new air filtration plant. The Doctor is initially dismissive… but when a man seems to appear out of thin air and die, his curiosity is piqued.

The Doctor soon discovers that the plant is connected to another time, another place. Stranded with Jo, the Brigadier and Benton on a distant planet, the Doctor discovers the terrible truth behind Breathe Industries' new technology. His oldest enemies, the Daleks.

Can even the Doctor and UNIT manage to save two worlds at the same time?