Osgood and the Zygon duplicate of her explain to viewers about the peace treaty signed between the human race and the Zygons. These events took place during the Day of the Doctor. They have become linked as a result and refer to each other as sisters. Through Operation Double, UNIT implemented a plan to relocate all Zygons into the human population by having them take the place of the duplicates of Britain. The 'sisters' have the "Osgood Box" which will e used if the ceasefire breaks; this is known as the Nightmare Scenario.

One Osgood is in a desert town and running away from the Zygons. She texts the Doctor with the message "Nightmare Scenario" before being zapped and taken captive by a Zygon. After receiving the message, the Doctor calls Clara but only gets her voicemail. In present day Britain, the Doctor tracks down the two commander Zygons at a school playground as they have taken on the form of two primary school girls. He pleads with them to not break the ceasefire but negotiations stall when another Zygon arrives, throws a smoke bomb and captures the two commanders.

Clara returns to her apartment and finds that she has 127 missed cals from the Doctor. She is going to call him back but then sees a boy on the stairs who is looking for his parents. She helps the boy get back to his apartment and the father takes him away kicking and screaming. Both the father and mother are exhibiting strange behaviour.

Clara meets up with the Doctor and together they go to UNIT where they learn that when one Osgood died, the other was overcome by grief. The ceasefire had started to break down. They receive a video message from the Zygons who are shown to force the primary school girl-Zygons to resume their normal form before they are destroyed.

The Doctor decides to go to Turmezistan to find the missing Osgood and keep UNIT from bombing and destroying it. Clara will keep looking after the ongoings in England, while Kate will head off to New Mexico to visit their stateside base. Unfortunately, the military members that are tasked with taking down the alien hostiles are overcome when the Zygons take on the appearance of the military members' loved ones. The Doctor is able to find Osgood trapped in a basement and frees her. Osgood warns that the Zygons are headed back to England.

Kate is being lead around a ghost town in New Mexico by a sheriff and observing the remains of dead Zygons in dumpsters that have been reduced to stacks of hair-balls.

Clara discovers a hive of Zygon duplicates that have been grown... except that's not how Zygons work. They kidnap the originals, so the Clara we've been watching for most of the episode is her Zygon duplicate. When Clara went to help the young boy, she was zapped and her duplicate took her place.

The Doctor and Osgood are questioning a Zygon and begin to piece together everything that has been happening. Meanwhile, Kate realises that the sheriff she has been led around with is actually a Zygon and is attacked and swapped out for a duplicate. Duplicate-Clara calls the Doctor and informs him that Clara and Kate are dead, before shooting a bazooka at the plane he is flying in on.

Doctor Who airs on Saturdays on BBC One.

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A battle is underway in an unknown war. A boy gets trapped in a field of "handmines," which are creatures that are hands with eye that drag their victims underground. The Doctor shows up and tosses his sonic screwdriver to the boy to try to save him, but is shocked when the boy says his name is Davros (the eventual creator of the Daleks).

Some centuries later, an alien creature named Colony Sarff who serves Davros, visits various planets in search of the Doctor. Sarff even visits the headquarters of the Shadow Proclamation, an intergalactic police force, as well as the planet Karn. Sarff has a message for the Doctor, that Davros is dying and that he "knows" and "remembers." Sarff visits Davros, whose condition is worsening rapidly, and delivers the bad news that he has not found the Doctor. Davros still possesses the Doctor's sonic screwdriver, and pushes Sarff to track down the Doctor's friends in the hopes that they will be able to help track him down.

Clara is teaching her class when she notices that a plane is frozen in the sky. As is turns out, planes worldwide are frozen in place. She is summoned to the Tower of London by UNIT to try to get hold of the Doctor. She also learns that the planes haven't stopped but are frozen in time, thus no one on board can be contacted. UNIT receives a message on a channel specifically created for the Doctor to use but the message is actually from Missy.

Clara goes to meet Missy at an open-air café, who admits to freezing the planes due to her pressing need to contact the Doctor. Missy shows Clara the Doctor's confession dial, which is the Time Lord equivalent to a last will and testament. They are typically delivered on the day prior to a Time Lord's death and had been sent directly to Missy, not Clara. Missy believes that this means that the Doctor believes he only has one day to live and she is concerned for him, believing him to be her friend albeit one with whom she has an immensely complicated relationship with.

Missy and Clara track the Doctor down to Essex in the year 1138 where the Doctor has spent the last three weeks "partying," and bringing forth a number of items out of place in that time period, including an electric guitar, tank, and fish. The Doctor is reunited with Missy and Clara but then Sarff appears, having followed the women. He reveals that he is a colony creature made up of one large snake plus several smaller ones, though he normally wears a robe to take on a more humanoid appearance. Sarff threatens the safety of everyone in the arena but the Doctor shuts his threats down, prompting Sarff to deliver Davros' message and returns the sonic screwdriver.

It's revealed through flashbacks that the Doctor had abandoned Davros when he was trapped in the handmines. The Doctor feels great shame for his actions and agrees to go with Sarff as his prisoner, despite Missy and Clara's objections, though they insist on coming along as prisoners too. Bors, the medieval man with whom the Doctor had been interacting, is revealed to be a Dalek puppet and procures the Doctor's TARDIS for the Daleks.

Sarff brings the prisoners to a space station, the Doctor is brought forth to Davros. Missy tells Clara that there's something off with the gravity and it doesn't feel artificial the way it would on a space station. They step outside and find they are on a planet and the Doctor begins to figure the same thing out as them: that they are on Skaro, the home planet of the Daleks.

Missy and Clara are captured by Daleks and brought to the room where the TARDIS is being kept. The Daleks intend on destroying it and Missy offers to help the Daleks utilise it instead of destroying it but they seemingly exterminate her before doing the same to Clara, though the Doctor had pleaded for her life but Davros says he can't control them. The Doctor expresses remorse for not destroying Davros when he had the chance and the loss of his friends. Davros wants the Doctor to admit that his greatest indulgence, compassion, is wrong.

Finally, we see the Doctor once again at the handmine field and meeting young Davros, who asks if the Doctor will save him. The Doctor says he will save his friend the only way he can, "Exterminate," while aiming a Dalek gun at young Davros.

Doctor Who airs on Saturdays on BBC One at 9PM.

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Clara is still trapped with the Cyberman in the 3W headquarters in St. Paul's Cathedral. She attempts to  trick the Cyberman into thinking she is The Doctor.

The Doctor is still in shock over the revelation that Missy is the Master. Citizens are snapping selfies with the Cybermen when UNIT, led by Kate Stewart, arrives on scene to apprehend the Cybermen. Unfortunately, St Paul's releases 91 Cybermen. These Cybermen fly off so that one will be over each city and major town in the UK. They will explode and release Cyber pollen over the graveyards and reincarnate the dead as Cybermen.

Danny is still in the Nethersphere. He learns that everyone in the "cloud" will be released back to their decomposing bodies through the use of rainclouds. They will be upgraded to Cybermen.

UNIT knocks Missy and the Doctor out but before he passes out, The Doctor passes on the message to guard the graveyards.

Clara is unable to convince the Cyberman that she is The Doctor but she is saved by Danny, who has been reincarnated as a a Cyberman.

The Doctor is on board a protected UNIT plane and learns from Kate that he has been nominated as President of Earth. He has control of the armies on the planet but is against this. He also believes there is no way to save Earth now.

Missy escapes and kills everyone in the room, including Osgood the scientist. The Doctor confronts Missy and learns that she arranged Clara and his being together. Missy has exploded the side of the aircraft, and the plane is on its way down having been attacked by Cybermen. Clara calls him on the Tardis phone, being at a graveyard with Danny, whom she has learned is a Cyberman. Danny wants her to delete his emotions, wanting the pain to stop. Clara pleads for his help.

Missy teleports out of the plummeting plane while the Doctor has to jump out of the plane and get to falling TARDIS. He gets inside and gets to Clara and Danny in the cemetery. The Doctor tells Clara not to turn Danny's emotions off, saying he will kill her after the fact. Danny insists he will not harm Clara. Danny convinces them to delete his emotions, saying he will be able to offer insight into Missy's plans from the core Cyber hive. After his change, he says that a second rainfall is coming that will convert living humans into Cybermen too.

Missy arrives at the cemetery and gives The Doctor control of the Cyber army as a gift. The Doctor instead gives it Danny, who leads the Cyber army into the sky where they explode and stop the second rainfall. Humanity is saved. Missy gives a set of coordinates that are apparently where Gallifrey is. Clara demands that The Doctor kill Missy so she doesn't repeat what she has done. The Doctor stops Clara, needing to save her soul. The Doctor prepares to kill Missy but a rouge Cyberman does it instead. The Doctor and Clara run over and find Kate lying on the ground, still alive. The Cyberman was a conversion of Kate's father and saved her from the fall out of the plane. The Doctor bids him goodbye with a salute, something he had always hoped to get The Doctor to do when he was still a human.

With Missy's control bracelet, Danny has the option to resurrect himself but he apologetically tells Clara that it can only be used once and has to keep his promises. He sends the child that he accidentally killed through instead. Clara is sad but agrees to help the child find their home. Clara meets with the Doctor and prepares to tell the truth about about Danny not being resurrected. But before she can, The Doctor says he understands that she is back with Danny now and won't want to be flying around anymore, and claims he has found Gallifrey using Missy's coordinates. In truth, Gallifrey was not there. They both maintain their lies to protect the other one when in actuality, they are both in a great deal of pain. The Doctor and Clara say their goodbyes. Clara thanks The Doctor for making her feel special.

In a mid-credits scene, The Doctor hears a knock on the door of the Tardis. He is shocked when Santa Claus comes in and asks what he wants this year.

Doctor Who will return to BBC One in December for a Christmas special, and in 2015 for series 9.

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