To the native swamp-warriors, Kroll was an angry, mythical god. To the money-grabbing alien technicians, Kroll was a threat to a profit-making scheme.

In their search for another segment of the Key to Time, the Doctor and Romana have to face the suspicion of the Lagoon dwellers, the stupidity of the technicians and, finally, the power of Kroll ...
 

The Doctor, Romana and K9 continue their search for the six disguised segments that make up the powerful Key to Time.

The TARDIS brings the Doctor and Romana to Delta 3, the third moon of the Planet Delta Magna. Romana is captured by the Swampies, the original inhabitants of Delta Magna, displaced from their home world by human colonists. They intend to sacrifice Romana to their god, the mighty Kroll.

The Doctor's search for his companionbrings him to a massive refinery in the middle of the swamp. The Doctor is mistaken for Rohm-Dutt, an amoral gun-runner who is supplying the Swampies with weapons to aid them in their plan to attack the refinery and force the workers to leave Delta 3.

The Doctor escapes the refinery in time to rescue Romana from the sacrificial alter, discovering in the process that Kroll is just a myth, a story to frighten children. They resolve to find the Fifth Segment and leave the moon avoiding the intrigues of the refinery workers and the attentions of the fanatical Swampies.

However, all is not how it initially seems. What is stirring up the floor of the swamp and can the instruments on the refinery really be correct in saying that it is five miles across? The Doctor realises that Kroll is more than a mere legend and its centuries-long slumber has been disturbed by the refinery. Now it is awake, the time travellers find that it is very large, and very hungry...

Originally transmitted: 23rd December 1978 to 13 January 1979

Running Time: 91 mins approx

Produced by Graham Williams 
Directed by Norman Stewart