The Nuclear first appeared opposite Tom Baker's Fourth Doctor in 1977's The Invisble Enemy by Bob Baker and Dave Martin. He was played by John Leeson (now best known as the Voice of K9)  - who returns in the role for this adventure! 

Revenge of the Swarm continues the story of Hector (Philip Olivier), now back aboard the TARDIS with the Doctor and Ace (Sophie Aldred)... but his past is lost to him.

Story details are as follows:

The Doctor thought he had defeated the microscopic Nucleus of the Swarm in his fourth incarnation. He was wrong. It survived within the TARDIS, and now it has brought it back to Titan Base, back to the point of its own creation. It has a plan that spans centuries, a plan which will result in the Nucleus becoming more powerful – and larger – than ever before.

To defeat it, the Doctor, Ace and Hex must confront the Nucleus within its new domain - the computer-world of the Hypernet, the information network crucial to the survival of the human empire. But if the Doctor is to save the day, he has to risk everything and everyone he holds dear...

Revenge of the Swarm is out in August and available to pre-order now.