In a final bid to regain control of the TARDIS' faulty control system the First Doctor is driven to experiment with a dangerous untried combination. With a violent explosion, the TARDIS blacks out and the crew find themselves trapped inside. A simple technical fault? Sabotage? Or something even more sinister? Tension mounts as the Doctor and his companions begin to suspect one another. What has happened to the TARDIS? Slowly a terrifying suspicion dawns. Has the TARDIS become the prisoner of some powerful fifth intelligence which is even now haunting the time-machine's dark and gloomy corridors?