Clara Oswald finally agrees to become the Doctor’s new companion in “The Rings of Akhaten.”
The Doctor takes Clara to a ceremony in which worshipers sacrifice their precious memories to a living world where all life originated. When Clara asks if their religion is true, the Doctor says in a somber voice “It’s what they believe.” The tone suggests the Doctor knows the true origin of all life. In fact, he explicitly states as much when offering his own memories in an attempt to overwhelm the entity.
The Doctor declares, “I watched the birth of the universe and I watched as time ran out moment by moment until nothing remained. No time. No space. Just me.”
As he continues he reveals a literal nuclear option as predicted.
“I know things. Secrets that must never be told. Knowledge that must never be spoke. Knowledge that will make parasite gods blaze,” he warns the creature.
It doesn’t seem he actually shares the secret knowledge as the parasite survives. Furthermore, it’s unknown if the Doctor suffers any memory loss from this encounter but if he does it could tie into Clara’s catchphrase, “Run you clever boy and remember.”
The parasite is only stopped when Clara offers her keepsake leaf, which has infinite possible memories. In the previous episode, Clara told the Doctor it was “page one” in her book of 101 Places to See. It turns out that the leaf brought Clara’s parents together.
Her father, Dave Oswald, was nearly hit by a car when the leaf flew into his face but he was pulled to safety by Ellie Ravenwood, Clara’s mother.
He gives her the leaf with a speech fitting a Time Lord:
This exact leaf had to grow in that exact way in that exact place so that precise wind could tear it from that precise branch and make it fly into this exact face at that exact moment. And if just one of those tiny little had never happened, I’d never have met you.
Ellie Oswald’s tombstone shows she died on March 5 2005, the day of the Nestene Consciousness invasion according to the missing posters of Rose Tyler which were dated the following day, March 6 2005.
Her mother’s death explains why age 16 is missing from Clara’s book, 101 Places to See. Age 23 is also missing, presumably because that’s when her father died in 2012. The only event that occurred on Earth that year was the Shakri invasion in “The Power of Three.” The Shakri are obsessed with the number 7, which is incidentally the amount of years between 23 and 16.