Danny Pink is dead-killed by a car accident. Totally distraught, Clara knocks the Doctor out with a sleeping pad, and threatens to destroy the TARDIS keys if he doesn’t go back to save her boyfriend. She seems to destroy the keys, but the Doctor states she’s actually been hypnotized to think she’s doing this (as he anticapted the sleep pad)-he wanted to know what was wrong, and help her through her grief.
He agrees to help her, to try to use the telepathic circuits, to perhaps find Danny in the afterlife, unsure of where they’ll go. They end up in a strange building, filled with skeletons in tanks of water.
There, they are ‘greeted’ by Missy, who claims to be a robot helping them around the facility. She kisses and embraces the Doctor, which kind of freaks him out and bit. She then summons Dr. Chang, who reveals that the skeletons are the preserved bodies of the dead, who are in some sort of exoskeleton (with the water causing a ‘see-through’ effect) as voices apparently have been discovered from an ‘afterlife’.
One of these ‘voices’ is Danny himself, who finds himself in a strange city “heaven”, although his body remains dead in ‘our’ world. He tries to communicate with Clara.
The Doctor however is very suspicious about the nature of the facility. Something’s not right here….and there’s something very strange and somehow familiar about the windows on the doors there…
The water drains, revealing that the skeletons are in fact the inner workings of Cybermen…
Also the “heaven” Danny is currently in, along with a number of other souls, is in fact a “Matrix slice”, Gallifreyan technology where their minds have been uploaded-and will now be downloaded-into the emotionless Cybermen army.
And the head of this army of Cybermen? Missy-not an android, but a person. Not just any person-but a person with two hearts-a “Time Lady”-one of the Doctor’s own species.
But not just any time lady. As the Cybermen pour out of 3W-in fact a dimensional expanded St. Paul’s cathedral-Missy reveals her identity to the Doctor-Missy, is in fact, short for Mistress…because due to a regeneration into a female body, she couldn’t very well call herself The Master anymore….
The Doctor is utterly stunned. One of his oldest enemies is back, is now a she, and has an army of undead Cybermen about to raise havoc.