Which 2006 two-part story first introduced the Cybermen in the revived series?

The 2006 two-part Doctor Who story that first introduced the Cybermen in the revived series was Rise of the Cybermen. Its second part, The Age of Steel, completed the story.

Broadcast during Series 2, the episode follows the Tenth Doctor, Rose Tyler and Mickey Smith after the TARDIS is damaged and they land in a parallel version of London. There, businessman John Lumic and his company, Cybus Industries, develop a process that places human brains inside metal bodies and suppresses emotion. This creates a new continuity for the Cybermen, separate from their classic-series origin on the planet Mondas.

The Cybermen had originally debuted in The Tenth Planet in 1966, so Rise of the Cybermen was a reintroduction rather than their first appearance in Doctor Who overall. Viewers sometimes identify The Age of Steel as the answer because it contains the climactic conversion and resistance storyline, but Rise of the Cybermen is the first instalment and the title normally used for the complete two-part story.