Travelling alone, the Doctor lands on an alien battlefield, where anachronistic planes fly over laser-firing soldiers. He sees a young boy who has wandered onto the battlefield, and is surrounded by hands reaching out of the wasteland-“handmines”. The Doctor hands him his sonic screwdriver, saying he’ll help.
He also asks the boy his name. The name however, chills the Doctor to the core.
It’s Davros. The Doctor has just offered to save a younger version of one of his greatest enemies-and the creator of the Daleks. This isn’t just any battlefield-It’s Skaro, in the final decades of it’s millennia-year war between the Kaleds and the Thals-which would reach it’s end with Davros creating the Daleks and taking control of most of the planet.
The Doctor quickly leaves, and journeys to Karn and later the middle ages, to mediate, and also puts together a confession dial. Meanwhile, a mysterious man called Sarff searches for the Doctor, telling people that Davros-in his old state and having survived the Stolen Earth incident-is finally dying.
Meanwhile, in the present, Clara is teaching a class when it turns out all the planes in the world have been frozen. Taken in by UNIT, Clara and Kate Stuart soon realize the source-Missy. Turns out she’s survived as well, and wants to find the Doctor-and she has his confession dial. Clara is forced to confront the female master, and the two are able to discover his location in time and space.
Turns out he’s mainly been hanging out in the twelfth century. Clara is surprised to see that the Doctor’s somewhat loosened up-he now sports more casual wear, including sunglasses, and performs with a rock guitar-on top of a tank no less-causing some major anachronisms.
Sarrf shows up-in fact not a man but a colony of snakes-and the Doctor-bothered that he abandoned the young Davros-agrees to be taken prisoner to visit the ailing mad scientist one last time. He also has the Doctor’s sonic screwdriver-which has been in Davros’s possession ever since. The trio are taken to a space station, and led to believe they’re only seeing Davros. As Davros taunts the Doctor however, it’s revealed they’re not in fact, in space-but on Skaro-somehow rebuilt after the Doctor seemingly destroyed it in his seventh incarnation.
A Skaro that is also populated by Daleks in a variety of casings-from their original silver to time war gold.
The Doctor then seemingly exterminate Clara, Missy, and the TARDIS. However, Missy and Clara in fact use the energy from the blasts to teleport (Also how Missy survived her apparent death by Cyber-brigadier last season) outside of the city.
The Doctor forces Davros out of his chair and confronts the Dalek council in that chair-one that’s shielded. He also enjoys some tea and threatens the Daleks to reveal Clara.
However, he’s quickly ousted from the chair by Sarff, and returned to Davros’s quarters, where they actually share a bit of a heart-to-hearts. Davros reveals he’s connected to every Dalek on Skaro, but even that can’t sustain him-although he offers the Doctor the power to once again, kill the Dalek race. Davros opens his eyes-which have been shut for many years following his injuries and are painful to open. He also reveals that there’s a prophecy-the prophecy of the hybrid-that two warrior races-presumably Daleks and Time Lords-would produce a hybrid-one that would ruin Gallifrey.
Meanwhile, Clara and Missy enter the Dalek sewers, which are mostly composed of decaying Dalek mutants who have been discarded from their casings. Triggering an alarm, Missy places Clara in the Dalek (ironically, we first met her that way, sort of) and poses as a Dalek prisoner.
Meanwhile, the Doctor agrees to give some of his regeneration energy to Davros to help keep him from dying. However, in doing so he falls into a trap-the energy is forcefully siphoned from him, and sent to every Dalek in the city. Davros was never dying-he wanted to lure the Doctor all along.
However, Missy is able to arrive and save the Doctor from Davros. Also, the Doctor’s regeneration abilities have also revitalized the decayed sewer Daleks-who now revolt against their encased younger selves and their creator. As the Doctor, Missy, and Clara run from the city, Missy states that the Dalek-encased Clara is a Dalek which in fact kills Clara, and the Doctor must kill it in revenge. Clara’s thoughts however are translated as typical Dalek speech. However, when the Clara/Dalek pleads for mercy, the Doctor is surprised that Daleks have a concept of Mercy. He then frees Clara, but tells Missy to run as he’s very peeved about her cruel trick. Missy is last seen surrounded by Daleks, but she has a clever idea-to use perhaps her teleport again using their energy blasts, or something else in mind. The Doctor then reassembles the TARDIS-turns out it wasn’t destroyed, just misplaced using an emergency system- and re-materializes it outside the city. Although he’s lost his sonic screwdriver, it turns out his sunglasses have a sonic function.
The Doctor then realizes where the concept of mercy came from, and goes back in time to the young Davros, where he uses a Dalek gun to get rid of the handmines surrounding the future mad scientist, letting him know the value of mercy-and in turn, giving the Daleks that concept too-even though of course they wouldn’t use it as much…