The TARDIS appears to die after the Doctor performs a recalibration. The TARDIS lands in London, but in an alternate universe. The Doctor works some magic, and says the TARDIS will be revived in about 24 hours, but he can't stop Rose and Mickey from investigating the alternate reality.
What they find is not what they expect. No one knows who the are, or knows them differently. Plus, they learn a vastly rich and powerful madman, bent on extending his crippled life, is experimenting on creating cybernetic robots out of humans. He is also intent on upgrading all humans into this cybernetic life form, to be known as... Cybermen.