The episode opens with Clara on the phone to Danny, she's going to tell him everything, but first she wants to make it clear how much she loves him, and that when she tells him she loves him she will never use those words for anyone else again. If that wasn't emotional enough for you tragedy strikes as Danny is hit by a car and dies. Clara can't come to terms with it, someone in her life having such a boring and ordinary death, so she calls the Doctor. She demands he take her to save Danny, threatening to destroy the TARDIS keys if he doesn't, the Doctor, though hurt by her betrayal, agrees. They come to a place where they see skeletons suspended in water and are greeted by Missy, who seems to be a greeting droid and then they are taught about the water by a doctor who works there; it has a refractive element that means you can only see organic matter within it, the skeletons all have a metal exoskeleton you can't see. Clara is allowed to speak to Danny, whose conciousness is in heaven as we've been seeing it this series, he attempts to save her from doing something stupid to be reunited with him; meanwhile Missy has the skeletons drain the water from around them revealing Cybermen and she reveals to the doctor that heaven is actually a piece of time lord technology storing the memories of the recently dead; the Doctor opens a door to find the building they were in was in London the whole time and tries to warn people of the incoming Cybermen and gets Missy to reveal her true identity.
Clara and Danny have some good material in this episode and they play up the tragedy of it well, but I'm kind of starting to see why people have a problem with Samuel Anderson as an actor, I've liked him most of the series, but he almost never puts anything into his performance coming of as kind of awkward and monotone. That said his death is handled well and the stuff with him in heaven raises a lot of uncomfortable ideas.
Overall I like a lot of this episode, but there's a lot of awkward comedy that I hate, Doctor Who never seems to be able to find a consistent tone; it jumps between the comedy and the drama but the comedy isn't funny and just distracts you from the drama. It's been a problem ever since it came back in 2005 and it will continue to be a problem; if you want to do a dramatic episode then just do a dramatic episode, you don't need to try comedy.