Eric Klieg was a member of the Brotherhood of Logicians.
Klieg, his colleague Kaftan and her servant Toberman accompanied Professor Parry's expedition to Telos to search for the tomb of the long-dormant Cybermen. He and Kaftan had funded the expedition in the hope they would be able to contact with the Cybermen and convince them to work with the Brotherhood to obtain power so the Brotherhood could at last attain its objectives.
The party eventually found the tomb after blasting away the side of the rock face.
Once in the tomb, Klieg set to work deciphering the logic patterns that granted access to the tombs themselves. After several failures, he managed it and most of the party went down, with Kaftan, Victoria Waterfield, Captain Hopper and the spacecraft crew staying behind.
When Kaftan shut the hatch behind them, the Second Doctor realised from Klieg's lack of distress that he was planning something, but allowed him to carry it out. Klieg claimed he could re-open the hatch using the controls next to the dormant Cybermen but, instead, he used them to wake the sleepers, knowing full well the panel wouldn't reopen the hatch.
John Viner tried to return them to their sleep, for which Klieg shot and killed him. This revealed his true colours, and he informed the party of what the true motives behind financing the expedition were. The Cyber-Controller listened to Klieg's proposition of mutual assistance before grabbing his arm and throwing him to the ground, declaring ownership over him and the others. Klieg was horrified when the Cybermen announced their plans to convert them into Cybermen themselves, and was even more horrified when the Cybermen promised they would purge them of their emotions.
Captain Hopper came into the tombs and used smoke bombs to create a distraction and allow the group to escape. Klieg was left behind and the hatch was shut in front of him. He knocked on it and, the lightness of this was recognised as human by the Doctor, so the party re-opened the hatch. The Doctor asked him if he still wanted to form an alliance with the Cybermen, and everyone discovered that despite his ordeal nothing had changed. Unwilling to have the schemers near them, Parry and the other expedition crew removed Klieg and Kaftan to the weapons testing room.
However, there they were able to obtain a working cyber-gun. Klieg believed it would give him bargaining power over the Cybermen but first wanted to kill the Doctor who was a threat to his plans. He fired upon him, but Jim Callum stepped in the way and was hit in the shoulder. Klieg used the gun to force the others to open the hatch. He shouted down that he demanded negotiations with the Cyber-controller, who emerged with the partially converted Toberman. Kaftan asked Klieg to obtain release for her servant, which he did as part of the negotiations, unaware that the controller desired this end, having control over Toberman's mind. After the Controller was revitalised, he had Toberman strike Klieg, knocking him unconscious.
When he awoke, he found Toberman, the Doctor and Jamie McCrimmon had re-entered the tomb, with the Cyber-Controller inactive on the floor nearby. Klieg woke the Cybermen again, who had returned to dormancy to conserve energy, still set on forming an alliance with them. While they awoke, the Doctor praised his genius and flattered him as to the greatness of his leadership in the future.
Klieg imagined himself as "master of the world". The Doctor replied that he "just wanted to make sure" he was, in fact, mad, and counted this delusion as proof. Klieg mocked them, telling him he would not kill them but leave them to the Cybermen, who would have a use for "parts of you". As he talked, a Cyberman grabbed him from behind and beat him to death. (TV: The Tomb of the Cybermen)