"The Age of Steel" is the sixth episode of the second series of the British science fiction television programme Doctor Who. It was first broadcast on 20 May 2006 and is the second part of a two-part story that was the first to feature the Cybermen since Silver Nemesis in 1988. The first part, "Rise of the Cybermen", was broadcast on 13 May 2006. It stars David Tennant as the Doctor, Billie Piper as Rose Tyler and Noel Clarke as Mickey Smith.

It focuses on events in a parallel universe London, where John Lumic plans to convert the whole world's population into Cybermen and the Doctor, Rose and Mickey Smith try to stop him. They are assisted by Mickey's parallel universe counterpart, Ricky, who is the leader of a resistance movement. At the end of the episode Mickey decides to stay in the parallel universe and says goodbye to the Doctor and Rose.

The episode was relatively popular as it was the climax to the first story to feature the Cybermen since the show's revival. It has an Audience Appreciation rating of 86.

The episode opens immediately after the events of "Rise of the Cybermen". The Cybermen have the Doctor, Rose, Mickey, and the Preachers surrounded. The Doctor uses the recharging power cell from the TARDIS to overload the Cybermen, disintegrating them. The group escapes with Pete Tyler, but Jackie is trapped inside and presumed dead. As they flee, Pete explains to the Preachers that he is "Gemini", the Preachers' secret source of information on Lumic. Pete had thought he was communicating with law enforcement but accidentally involved the Preachers instead. From his hovering zeppelin, John Lumic orders the Cybermen to activate the EarPod devices and use them to control the people of London and bring them in for conversion.

...
Wikipedia