Doctor Who: The Three Doctors by Terrance Dicks
A mysterious black hole is draining away power from the Universe. Even the Time Lords are threatened. The Doctor is also in trouble. Creatures from the black hole besiege UNIT Headquarters. The only person who can help the Doctor is... himself.
The Time Lords bring together the first three incarnations of the Doctor to discover the truth about the black hole and stop the energy drain.
The Doctors and their companions travel through the black hole itself, into a universe of anti-matter. Here they meet one of the very first Time Lords - Omega, who gave his race the power to travel through time. Trapped for aeons in the black hole, he now plans to escape - whatever the cost.
This novel is based on a Doctor Who story which was originally broadcast from 30 December 1972 to 20 January 1973.
Featuring the first three Doctors as played by William Hartnell, Patrick Troughton and Jon Pertwee, together with Jo Grant and the UNIT organisation commanded by Brigadier Lethbridge-Stewart