The Doctor and his companions arrive on a large Ark in space and attempt to assist the residents. Unfortunately, they fall seriously ill with a common cold; a virus to which they've never been exposed before. The Doctor manages to find a cure and they leave, only to materialize in the same place but centuries later. The simple creatures who were servants in the past have revolted and seized control of the Ark. Soon a plot to destroy the Ark and all it's human inhabitants is exposed but can they find the bomb in time?