Surgeon Lieutenant Harry Sullivan is supposed to be on holiday. But his arrival in Calcutta is intercepted by someone from the local branch of UNIT - a staff member called Naomi Cross.

A businessman in the area has mysteriously cancelled the opening of his new hotel... and local villagers have reported sightings of 'devils' in the swamps. Devils that look very familiar from the UNIT files...

The businessman requires medical aid so UNIT have decided to send him a doctor. Unfortunately, when Harry and Naomi arrive on site they find a different Doctor has got there ahead of them... And that's just the start of their troubles.