The Runaway Bride 3.1
The Earth did not form in the way you think it did. You see, there was this alien species called Racnoss, and the Earth formed around their dying spaceship after the Timelords had destroyed them, but there is one left: The Empress. The Doctor (#10, Tennant) meets Donna Noble (Catherine Tate) when she arrives unexpectedly in his TARDIS, having been teleported from the alter at her own wedding. They discover this is because her hubby, Lance, had been drugging her with huon particles, under orders from the Racnoss Empress, a giant spider from another planet.
In the pit beneath Donna’s workplace where the Empress has been hiding out, Donna is captured into a giant spiderweb and the Empress’ offspring begin to come to life! But don’t worry, the Doctor gets his hands on some explosives, and explosives make everything all better, don’t they? With it, he kills the offspring, and the Empress and her ship are shot out of the sky by the humans.
In the end, the Doctor invites Donna to come traveling with him. Although she warns him that he shouldn’t be alone, she declines his offer, leaving him in solitude.
Kill the Moon 8.7
The Earth’s moon did not form in the way you think it did. You see, there was this alien who laid an egg and we mistook it for a moon. The Doctor (#12, Capaldi) doesn’t know the species and suspects this is the only one left. As the gigantic offspring begins to crack its shell, a secondhand rocketship driven by thirdhand astronauts arrives on the moon with explosives, because explosives make everything all better, don’t they? In an attempt to take Clara’s student Courtney to the moon, the TARDIS finds itself on this rocketship and the three are left with the burden of helping the crew (1) make the decision of whether or not to murder the giant baby in order to stop the flooding on Earth and (2) prevent them from getting eaten by the giant spiders from another planet. That’s right–more of people getting wrapped up in giant spiderwebs! But lest you think you’ve seen this formula before with The Runaway Bride, the spiders aren’t really spiders. They are germs living on the flesh of the giant baby in the egg. Because that’s how germs work, don’t they? The bigger the animal, the bigger the germs?
Anyway, Courtney busies herself posting astronaut photos on Tumblr while the Doctor buggers off in his TARDIS, leaving Clara to decide whether or not to murder the baby and blow up the moon, or whether to let it hatch with the possibility of damaging the Earth with shards of eggshell (not to mention whatever threat a giant space dragon might be to the Earth). Although the Doctor had no problem blowing up and flooding out the Racnoss’ offspring, his vote is clearly to save this one, and eventually Clara makes that decision as well. The Doctor made it clear that he didn’t know whether the Earth would be safe from the hatching, and so possibly for the first time, he doesn’t put humans first and goes with a less genocidal plan, albeit in a passive, “let the humans decide” way. It is not a total flip on its head change for the Doctor but it is certainly a shift, and it is surprisingly not a darker one.
When the baby lives and does not destroy the Earth, the Doctor reveals that now he can remember the future, that this was the day that humans looked up and saw aliens that they didn’t want to kill and that was the first step toward their journey into the stars, and the reason why the human race survives forever.
In the end, Clara’s fury at being left alone to make a decision she almost got wrong leads to an argument with the Doctor that has to be the best moment of season 8 so far. She leaves the TARDIS, vowing never to return, leaving the Doctor in solitude.