In a final bid to regain control of the Tardis's faulty control system the 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?
