When we last left off, Missy revealed herself as the Master just as the Cyberman army roused itself from its slumber. Usually, this would be the point where Cybermen open fire on every major world city until the Doctor saves the day. But instead they surprise everyone, including the Doctor, by... doing absolutely nothing. The Cybermen stand stock still on the streets, which even prompts a few civilians to take pictures with them. It's a little confusing, but makes clear that Missy/The Master has something sneakier in mind. Not that she gets much of a chance to explain it, because their arrival prompts UNIT to rush onto the scene.
It turns out that UNIT has actually been conducting its own investigation of the 3w corporation, so when the Cybermen appear they're fully prepared. It's also a great chance to see some welcome faces like Kate Lethbridge-Stewart and Osgood, who haven't appeared since "Day of the Doctor". Osgood's attire has noticeably changed, having dropped the Fourth Doctor's scarf in favor of the Eleventh Doctor's shirt and tie combo. But given her obvious Doctor crush and the events of "Day", that's highly appropriate, and a nice touch for fans missing a dash of something "cool".
But the Cybermen aren't interested in attacking UNIT, at least not yet. Instead, they launch themselves into the atmosphere and detonate, spreading rainclouds across the Earth filled with nanites. As it rains, all the dead bodies Missy interacted with for countless generations immediately awaken as Cybermen.
Last week's explanation of Heaven was certainly creepy, although it still strained our disbelief. The true story makes a little more sense in context to Doctor Who lore: After the Doctor saved Gallifrey last season, Missy was able to escape with her own Tardis and traveled into ancient human history to create the concept of an afterlife. By tricking wealthy humans across time into giving up their wealth for immortality (think of how the Pharaohs placed immense importance on preserving their bodies), Missy gained enough wealth to support the 3w corporation and start a Cybermen army. Once established, every single person who died found their mind downloaded into a digital cloud, where they were coerced into deleting their emotions so the Cybermen didn't have to.
Take a moment to let that sink in. This episode literally turns every single dead person on the planet into a Cyberman, including (I assume) your own family and ancestors. That not only makes "Death in Heaven" impressively large in its scope, but leaves a darkly personal touch lingering behind the surface. Every single graveyard and morgue now has Cybermen emerging from them, which I suppose is the closest Doctor Who will get to a Romeroesque zombie apocalypse. But it also reflects the personal stakes of the finale, especially as events move forward.
So how exactly did 3w manage to download every single deceased human mind across time and space? Who cares! That's how.