In the mysterious world of the Nethersphere, plans have been drawn. Missy is about to come face to face with the Doctor, and an impossible choice is looming… ‘Death is not an end’, promises the sinister organisation known only as 3W – but, as the Doctor and Clara discover, they might wish it was.
Clara is talking on her phone to Danny Pink, ready to admit she has been travelling with the Doctor and that she sincerely loves Danny, when the line goes silent. A stranger picks up and sadly informs Clara that Danny was hit by a passing car and has died.
She struggles with his death for days, unwilling to let her emotions go. She ultimately schemes to force the Doctor to change the past. She suggests that they visit an active volcano, secretly collects all seven TARDIS keys and, after the TARDIS arrives, slaps a sleep-inducing patch on the Doctor’s neck. Once the Doctor wakes up at the volcano, locked out of the TARDIS, Clara throws the keys one by one into the lava – the only way to destroy them. She says she will not stop unless the Doctor agrees to save Danny, despite his claim that it would create a time paradox. The Doctor, however, refuses and Clara destroys all of the keys, tearfully saying she would do it again. The Doctor eventually reveals that the entire scenario was only a hypnotic vision of hers. The patch did not work on him; instead, he had used it on her to learn why she was upset and to see how far she was willing to go. He then offers—as a friend, despite her betrayal of him—to see if they can locate Danny in the afterlife and rescue him.
The Doctor has Clara link with the telepathic interface of the TARDIS to focus on Danny’s location. They arrive at a strange mausoleum which holds several tanks of human skeletons underwater. They are greeted by Missy, who says she is an android receptionist for the 3W facility. Missy behaves very forwardly with the Doctor, even kissing him passionately as a gesture of welcome, which makes both him and Clara uncomfortable. Missy then introduces them to Dr. Chang, one of the scientists working there. Posing as a government official, the Doctor inquires about the facility. Dr Chang reveals that the skeletons do not drift apart because they are submerged in a substance called dark water, which only allows organic material to be seen, and that the bodies are encased in supportive exoskeletons. He further explains how the facility was founded based on the discovery of voices of the recently deceased within white noise broadcast signals. The facility’s name, 3W, is short for the “3 words” that were heard repeatedly in the broadcasts: “don’t cremate me”. The Doctor remains dubious of the 3W facility’s intentions.
During all of this, Danny has regained consciousness in a spartan office and been greeted by Seb, who reveals he is in the Nethersphere, a giant city enclosed within a sphere. Seb explains that Danny has died and helps him to adjust to this revelation. Seb reveals that Danny is still tied to the state of his body — it is probably being kept in a refrigerated morgue, as he constantly feels cold. As part of his orientation, a meeting is arranged between Danny and a young civilian boy that he had accidentally killed while he was a soldier in Afghanistan, the guilt having plagued Danny for years. Danny tearfully tries to offer his apologies, but the boy refuses and runs off. As Seb and Danny talk, Seb receives a notice that Clara is trying to contact Danny—arranged by Dr. Chang from the 3W facility. The Doctor leaves with Dr. Chang to investigate the facility further while Clara takes the call, asking questions to try to confirm that she is really speaking to Danny. However, Danny cannot provide conclusive information. Clara says that she will do anything to be with him once she is convinced. Danny, not wanting her to die to join him in the afterlife, tricks Clara to end the call. This final act renders him emotionally distraught. Seb then offers him the option of deleting all of his emotions.
Towards the end of the episode, Missy reveals her true identity as the Master to a horrified Doctor. The climax was removed from all preview copies of the episode, and the actors involved recorded their lines in ADR to hide this revelation from spectators.
Meanwhile, Missy instructs the skeletons to rise and begin draining the tanks. As the Doctor and Dr. Chang enter, Chang reveals that Missy is the facility’s supervisor, not its receptionist. Disappointed with Chang, Missy kills him with a handheld device. As the tanks start to drain, the skeletons are revealed to be Cybermen. Missy shows the Doctor a spherical device on the ceiling called the Nethersphere, a Gallifreyan Matrix Data Slice in which the consciousnesses of the deceased are held and prepared to be inserted into new Cybermen soldiers after their emotions are deleted. Stunned, the Doctor realises that Missy is really a Time Lady having two hearts, which he had first noticed from their earlier contact. As the tanks finally drain and open, the Doctor races out of the facility and finds that it is inside St Paul’s Cathedral in the middle of contemporary London. He tries to warn away civilians, but Missy calls out that his warnings are the ravings of a lunatic. The Doctor insists on knowing who Missy is. She reveals that “Missy” is short for “Mistress”—since in her current form, she cannot really call herself the Master.
The episode ends with those Cybermen beginning to march on London. The Doctor is shocked by Missy’s identity, Clara is trapped in Dr. Chang’s lab with another Cyberman. Danny is about to activate the deletion of his emotions when he sees in the reflection of his afterlife iPad the young boy that he had accidentally killed.