Which Doctor Who serial first introduced the Cybermen?

The Doctor Who serial that first introduced the Cybermen was The Tenth Planet.

Broadcast in four parts from 8 to 29 October 1966, the story is set at Snowcap Base in Antarctica in the year 1986. The First Doctor, played by William Hartnell, discovers that Earth is being threatened by Mondas, a long-lost twin planet that has returned to the Solar System.

The Cybermen are revealed as former inhabitants of Mondas who replaced much of their biology with machinery. Their conversion process and rejection of emotion established the ideas that would define them as recurring Doctor Who villains. Writer Kit Pedler and script editor Gerry Davis created them partly in response to contemporary advances in prosthetic and transplant medicine.

The Tenth Planet is also important because it was Hartnell's final regular story and introduced regeneration. The Tomb of the Cybermen, broadcast in 1967, was their second televised appearance—not their debut. The fourth episode of The Tenth Planet remains missing, though it has been reconstructed using animation and surviving material.