Doctor: Peter Capaldi (Twelfth Doctor)
Companion: Jenna Coleman (Clara Oswald)
Samuel Anderson – Danny Pink
Ellis George – Courtney Woods
Hermione Norris – Lundvik
Tony Osoba – Duke
Phil Nice – Henry
Christopher Dane – McKean
Directed by Paul Wilmshurst
Written by Peter Harness
Script editor David P Davis
Produced by Peter Bennett
Executive producer(s) Steven Moffat
Incidental music composer Murray Gold
Series Series 8
Length 45 minutes
Originally broadcast 4 October 2014
After the events of "The Caretaker", Clara Oswald warns the Doctor that her student, Courtney Woods, is still interested in his TARDIS and is trying to clean up the mess inside it she made previously. As they enter the TARDIS, the Doctor stops Courtney from cleaning and offers to take her and Clara on a trip in the time machine.
They arrive in 2049 aboard a space shuttle filled with nuclear devices on a crash course for Earth's moon. They survive and meet the shuttle's crew, led by Captain Lundvik. Lundvik explains they are on a suicide mission to destroy the moon; some years past, the moon's gravitation influence suddenly changed, causing massive high tides on Earth wiping out much of humanity, and destabilising the orbit of many of mankind's artificial satellites. Lundvik, who had wanted to become one of Earth's space-faring astronauts, offered to go on this mission to save humanity. Privately, Clara asks the Doctor about this point in time, knowing the moon exists in Earth's future, but the Doctor himself is not clear, as this is a point of time in flux.
They travel to a nearby Mexican lunar colony, finding it covered in cobwebs and the colonists dead, entombed in similar webbing. The Doctor confirms the colony's readings that the moon has gained over one billion tons of additional mass. They are attacked by a spider-like creature that kills Lundvik's crew, but the disinfecting spray Courtney brought kills the creature; the Doctor compares it to germs, and wants to study it further. After seeing Courtney safe to the TARDIS, they investigate a crevasse where thousands of the spider-like creature wait, and the presence of amniotic fluid nearby. The Doctor surprises Clara and Lundvik by diving into the crevasse, promising to meet them later.
The shuttle, with the TARDIS inside, fall into a crack. The Doctor returns and the three take shelter at the colony. The Doctor asserts that the moon is really a giant, 100-million-year-old egg, ready to hatch, the spiders being the equivalent of sterilising microbes on its surface. The Doctor gets Courtney to join them in the TARDIS. Lundvik becomes even more insistent to blow up the moon, unsure of the nature of the creature that might hatch, which angers the Doctor. The Doctor takes the TARDIS and abandons the three of them, but assures he will be back once they have made a decision. Lundvik prime a remote trigger for the nuclear bombs set on a timer, intent on finishing her mission. Lundvik, Clara, and Courtney argue what to do, and come to a conclusion to let the population of Earth decide. As shown in medias res at the start of the episode, Clara makes a plea over broadcast channels for Earth to decide the fate of the creature by leaving their lights on to allow the creature to live, or turning off their lights if they should destroy it. Over the next hour, they witness the lights on Earth turn off. Clara changes her mind at the last second and manages to stop the countdown on the remote detonator. Shortly thereafter the Doctor arrives, assured that they have made the right choice, and evacuates them from the moon as it starts to crumble.
Safely on Earth, they watch as the winged creature inside the moon breaks free of the shell and flies off, but not before laying another egg that becomes a replacement moon for Earth. The Doctor reveals that the sudden interest in the moon will reinvigorate Earth's space program, and Lundvik will get the opportunity to realise her dream. The Doctor returns Courtney and Clara to their rightful time; Clara angrily accosts the Doctor for forcing her to make the decision on the fate of humanity, while the Doctor asserts he cannot be allowed to make those choices himself. Clara tells the Doctor she does not want to see him again, and takes comfort with Danny Pink, though Danny sees she is still not ready to forget the Doctor. Clara returns to her flat that evening and looks reflectively at the moon.