The TARDIS attaches itself to a space liner after Turlough, still under the Black Guardian's influence, damages its controls. The Doctor and Nyssa meet two space pirates, Kari and Olvir, who have come on board the liner in search of plunder, while Tegan and Turlough get lost in the infrastructure. The liner docks with what appears to be a hulk floating in space. This is Terminus, which claims to offer a cure for Lazar's disease. It is crewed by armoured slave workers, the Vanir. The cure is administered by a huge, dog-like creature known as the Garm. Nyssa, who has contracted the disease from sufferers transported aboard the liner, discovers that the cure - involving exposure to radiation - does actually work.