Part Four
Series Episode Number: 503World Premiere: Sat 10 Feb 1979 - 6:27pm GMT [BBC One] (United Kingdom)
Running Time: 25 minutes 9 seconds
8.60 Million (UK)
Chart Position: 60 () (UK)
Recorded on Wed 22 Nov 1978 in Television Centre 3
With the Marshal poised to fire upon Zeos and the Mentalis computer on a countdown to destruction, the Doctor resorts to desperate measures to prevent a cataclysm.