Courtney Woods

After recent events that occurred at Coal Hill School, in which The Doctor took on the role of the school's caretaker (see "The Caretaker"), Clara Oswald warns The Doctor that her student, Courtney Wood, has become self-destructive as a result of The Doctor telling her she isn't special. Courtney is also trying to clean up the mess that she made during her first trip in the TARDIS. As The Doctor and Clara enter the TARDIS, The Doctor stops Courtney from cleaning and offers to take her and Clara on a trip to the Moon so that Courtney can become the first woman on the Moon.

But instead of landing on the Moon, in 2049, the TARDIS materialises inside a Space Shuttle filled with nuclear devices on a crash course with the Moon. They survive the crash and while The Doctor is curious as to higher-than-expected gravity they are experiencing, the three are met by Captain Lundvik and two other astronauts (Duke and Henry). The Doctor deduces that the Moon's increased gravity is being caused by added mass.

Lundvik explains that they are on a suicide mission to blow up the Moon; she explains that years prior, there was a sudden mass high tide, wiping out a fraction of humanity and taking out many of humanity's satellites. A Mexican colony on the Moon had reported changes in the Moon that led scientists to believe the problem existed there, and by destroying the Moon, they will be able to save humanity from further harm.

Courtney and Clara

The group then sets off for the Mexican colony, which they find oddly covered in spider webs. Lundvik sends Henry to set up the bombs as they explore inside the colony. There they find the Mexician colonists entombed in more spider webs. The Doctor finds readings they had taken, showing seismic activity on the surface, and photos showing the deformed landmasses as the Moon begins to disintegrate. He also deduces that there has been an increase of the Moon's mass by over 1 billion tons, explaining the sudden tidal shift on Earth.

Meanwhile Henry is attacked by a large spider while investigating an opening on the surface. At the same time another of the spider creatures enters the colony base and begins menacing The Doctor, Clara, Courtney and Lundvik, and kills Duke. Courtney though becomes separated from the others but is able to kill the creature with a bottle of cleaning fluid she had brought with her from the TARDIS. On examining the dead creature The Doctor realises that these creatures are like germs. Clara then questions The Doctor about the fate of the Moon telling him that she knows the Moon is not destroyed, having seen it above Earth in the future. But The Doctor says this is a fixed moment in time and so he does not know what happens to the Moon which could be destroyed so affecting Humanity's future.

The Doctor

After returning Courtney back to the safety of the TARDIS, The Doctor leads the group back outside to investigate one of the cracks on the surface caused by the corrosion. There they find Henry's corpse and also thousands more of the spider-like creatures inside the crack. The Doctor is attacked by another spider but it is repelled by the deterring effects of sunlight. Nearby in a large hole in the Moon's surface The Doctor discovers amniotic fluid. To the others' surprise, The Doctor tells them that he will be back before diving into the hole.

More tremors occur and Clara and Lundvik witness the Space Shuttle, with the TARDIS still inside, fall into a large crack that appears on the surface. Unable to do anything they return to the colony to wait, with Lundvik preparing to set the timer on the nuclear detonators despite Clara's assurance The Doctor will be back. The Doctor then appears soon after and he explains that the Moon is really an egg and the creature inside is ready to hatch after growing for millions of years. While the spider-like creatures are the equivalent of microbes on the surface of the egg. Lundvik immediately asserts that they should still destroy the Moon and kill the creature to prevent any possible harm to humanity the creature may cause, but Clara refuses to accept this solution even though Lundvik reasons that the Moon's disintegration is causing the catastrophic weather effects on Earth, and needs to be destroyed to save the world.

Courtney, who is still inside the TARDIS, wants to return to the colony so as to be a part of the debate. The Doctor therefore instructs her over her phone how to bring the TARDIS to him. Not being from Earth or the Moon, The Doctor chooses not to interfere, insisting that only Humans can decide the future of their planet and so leaves Clara, Courtney and Lundvik stranded on the Moon, forcing them to decide what to do in the short time left before the egg hatches.

Moon Arrival

The three of them attempt to come to a decision but none of them can make the choice about saving humanity at the cost of sacrificing the life of the creature in the egg. As the Spider Germs begin to swarm over the surface of the Moon, Lundvik is able to make contact with Earth and Clara uses a simultaneous television broadcast to plead to humanity to help them make their choice, by either turning off their lights to destroy the Moon or keeping them on to let the creature live. Lundvik then initiates the countdown on the nuclear devices as they wait for Earth's response which they will be able to see on the Moon. However, despite all the lights on Earth going out Clara intervenes and stops the countdown at the last second just as The Doctor returns, knowing they have decided to allow the creature to live, and rescues them from the Moon as it starts to crumble.

Back on Earth, the four of them watch as the Moon disintegrates and the creature that was inside fly away. At Clara's insistence, The Doctor reveals that because of the brief re-interest in what happened with the Moon, humanity would be rekindled to travel to the stars, helping them to spread across the universe, and then assures Lundvik that she will now have a real space program to lead. They then witness a new Moon - a freshly-laid egg by the creature - appear in Earth's orbit.

After they return Lundvik and Courtney to their proper time and place, Clara demands to know what The Doctor knew in order to leave them with such an important decision, threatening him that she will smack him so hard, he'll regenerate. When The Doctor explains why he abandoned her, Clara strikes out and stands up to him. She expresses her anger for him leaving them behind on the Moon, and becomes tearful, then even more angry when The Doctor gives patronising responses to her feelings. Realising how hurt Clara is The Doctor tries to make it up to her, but Clara is too furious to listen. She tells him that she never wants to see him again, travel through time and space with him again and ends their friendship, leaving The Doctor all alone in the TARDIS.

Back at Coal Hill School, she is comforted by Danny Pink. She explains to him that she is done with The Doctor but Danny tells Clara her relationship with The Doctor is not over, because he can still make her angry and that she is really not ready to move on. When asked how he became so wise, we learn that he left the army under circumstances that he refers to as a really bad day. Clara returns to her flat and looks out onto the Moon, which has not hatched yet, shining in the night sky.