First Transmitted 2 November - 21 December 1968
When the Doctor, Zoe (hurray!) , and Jamie investigate the mysterious dissapearance of an eminant scientist, their search takes them to International Electromatics, a powerful firm run by the sinister Tobias Vaughn. Under surveillence by the Brigadier's top secret United Nations Task Force, it soon becomes apparent that there is more to Vaughns activities than meets the eye. Just who are his mysterious allies? What is lurking down the sewers of London? Is the Doctor ready to face an old enemy once more?