"There's quite a difference, isn't there, between what was and what should have been. There's an awful lot of one, but there's an infinity of the other."
— The Doctor
The first broadcasted Doctor Who episode by Promoted Fanboy Neil Cross, who got the job by mentioning Doctor Who for years in every BBC meeting.The Doctor quietly watches (and readsThe Beano) as Clara's parents meet for the first time. A leaf blows into Clara's dad's face and he stumbles into the road, where her mother saves him from being hit by a car. The two fall in love, become Clara's mother and father, raise her with kindness and sweet stories. The Doctor is always there, just to watch her grow up. When Clara's mother dies, the Doctor is there at the graveyard, just looking at the scene from a distance. Clara is, for all intents and purposes, a completely normal human being.On the day after her previous adventure, the Doctor arrives to come pick Clara up. Clara wants to see something awesome, so the Doctor whisks her off to the gorgeous inhabited rings of the planet Akhaten, where the Festival of Offerings is in full swing. Wandering off at the marketplace, Clara meets Merry, the very young Queen of Years, as the pilgrims and natives prepare for the ceremony. Merry is terrified of her role as a ceremonial singer, thinking she'll screw up despite having been extensively taught every song and story known to her culture. Clara encourages her to go through with it, remembering how her own mother raised her to be courageous, and Merry agrees to perform the hymn for "Grandfather", the elder god. However, said god very unexpectedly interrupts little Merry's song and drags her through the air towards its tomb. The Doctor and Clara stop being tourists and leap into action, following suit on a space motorbike.The Doctor sonics open the tomb door and becomes forced to seal himself, Clara, Merry and a monk (frantically singing sacred lullabies) in together with a tremendously scary mummified vampire, who promptly wakes up and starts attacking its own cage. Merry insists on making a Heroic Sacrifice and letting it feed on her memories — all the songs and stories she's been taught for this purpose. But the Doctor explains to her the sheer improbability of someone as unique as a living person being born in the first place (glancing at Clara while he talks) and the pointlessness of sacrificing something as beautiful as her own life.Merry is still intensely scared, though, and she fully blames Clara for convincing her to go through with the ritual, even though no one knew it would lead to this. While Merry uses her knowledge of her culture's lore to improvise an escape, the thing breaks through its glass and lets out a shrill yell, which triggers a beam that makes contact with the planet, after which the mummy falls unconscious. As it turns out, the mummy isn't the Old God. It was the Old God's alarm clock. The Old God is the entire planet.The Doctor asks Clara to get Merry to safety on the air bike, while he stays behind to confront the living, monstrous planet. Merry takes up her old spot on the ceremonial dais again and begins to sing, hoping to calm the beast. The Doctor exposes himself to it, voluntarily letting the thing Mind Rape him and showing it every single one of his memories: the Time War, the beginning of life and its ending, the death of entire universes. Entire worlds that laughed in the face of physics and relied on the mind of a madman. The monster isn't quite sated yet, so while the Doctor, crying by this point, struggles to keep standing up, Clara rushes back into the tomb and offers the monster her leaf: not just its memories, but Clara's grief at what might have been if her mother hadn't died. Since the thought of innumerable possibilities is stronger even than the Doctor's 1200 years of life, the planet gorges itself on Clara's grief and dies.In the aftermath, the Doctor explains that Clara reminds him of someone who died. Clara tells him she won't be a Replacement Goldfish for whomever it is he lost, not realising he's talking about her.
The Doctor: I walked away from the Last Great Time War. I marked the passing of the Time Lords. I saw the birth of the universe and I watched as time ran out, moment by moment, until nothing remained. No time. No space. Just me! I've walked in universes where the laws of physics were devised by the mind of a MAD... MAN. I've watched universes freeze and creations burn. I have seen things you wouldn’t believe. I have lost things you will never understand! And I know things. Secrets that must never be told. Knowledge that must never be spoken. Knowledge that will make parasite gods BLAZE!SO COME OOOOON THEN! TAKE IT! TAKE IT ALL, BABY! HAVE IT! YOU HAVE IT ALL!.
Batman Can Breathe in Space: The entire episode is spent on asteroids that should never hold an atmosphere, the habitable parts of these asteroids extend onto the surface and into empty space, and there are several sequences where the characters ride a space-moped without helmets or space suits. It's reasonable to assume there's an artificial atmosphere, but this is not elaborated on in the episode.
Conspicuously Light Patch: Near the episode's beginning, there's a shot of a tree with one big red leaf, and many smaller yellow leaves. Guess which one ends up being "the most important leaf in human history"?
The Doctor mentions having come to Akhaten before, with his granddaughter. (Which makes it the first explicit mention of Susan Foreman in the revived series.)
He also again mentions having seen the birth of the universe. And the end of it (which he's technically been to at least twice).
Clara tries to get into the TARDIS, but it stops her, and after a while she randomly comments "I don't think it likes me." The TARDIS possibly locked out Clara on purpose, just like she went to the end of the universe to try and stop Jack from holding onto her during "Utopia".
Indy Hat Roll: The Doctor does one under the closing tomb door; rolling back to retrieve his sonic screwdriver.
Internal Homage: The Doctor's speech to the monster is very reminiscent of the Eighth Doctor's speech in "Phobos". Neil Cross' other series 7 episode, "Hide", made the link more explicit with many direct references to the Eighth Doctor's audios.
Kill the God: What the Doctor and Clara have done at the end is effectively kill these people's God. The ramifications for these people on a cultural and spiritual level aren't dealt with, considering how much of a bond it created between them and we already (possibly) saw how the monk in the Temple dealt with this loss.
Load-Bearing Hero: The Doctor manages to open the door to the temple with his sonic screwdriver, but has to keep it there to prevent the door from closing. Paradoxically, he still seems to feel the weight of the door despite not actually touching it, which he lampshades a few times.
Victorian Clara's gravestone in "The Snowmen" says she was born on November 23, the date "An Unearthly Child" aired in 1963. She also lived to age 26, the same age as classic Doctor Who when it was cancelled in 1989. Clara's mother died on March 5th, 2005, the day "Rose" (the first episode of the new series) takes place.
No Biochemical Barriers: Clara chomps on an alien fruit before the Doctor has finished scanning it to make sure it's safe. The Doctor doesn't even bother reprimanding her, presumably as he's got exactly the same attitude to potential danger.
No Endor Holocaust: The Doctor later mentions all of the people having been saved, in spite of the destruction of a planet, suggesting that the loss has somehow not been insurmountable for them or that whatever system is able to maintain a unified interplanetary atmosphere also protected the planets.
Our Vampires Are Different: The Doctor calls the Alien God a vampire, but only in the sense that it's a parasite. Turns out the real Old God isn't even remotely humanoid.
Phlebotinum Overload: The Old God feeds on the life experiences of others. But when Clara offers it "the most important leaf in human history", containing not only the experiences of its owner but all the experiences they could have had, it implodes. The Doctor tried to invoke this with his own memories, and came pretty close, but the planet survived that.
Red Herring: The Doctor mentions visiting Akhaten with Susan (first time she was explicitly referred to in the new series), and then we find out that the god of Akhaten is called "Grandfather". At first, it looks like the aliens are worshipping the Doctor.
Saving The World With Art: The people of Akhaten balieve that a song must always be sung in order to keep an angry god asleep, and if it ever ceases he will awake and devour them all.
Scenery Porn: The Rings of Akhaten are gorgeous and shown in great detail. Of course, Clara did ask to go "somewhere awesome", so why disappoint?
Screw This, I'm Outta Here!: The temple monk presses a button on his wrist device and disappears into thin air. This might have been suicide, but his dialogue is somewhat ambiguous about his plans.
Neil Cross: Marcus (Wilson, producer) phoned me up and said, "We've always wanted to have a speeder-bike like in Return of the Jedi and we know how to do it inexpensively, so can you get one into the story?"
Also, the bazaar reminds one of the famous Cantina scene in A New Hope.
One of the races the Doctor claims makes up the crowd is the Hooloovoo. It appears to be an In Name Only reference though (possibly an attempt at trolling by the Doctor) since the Hooloovoo are described as "a hyper intelligent shade of the colour blue", whereas it's a human-looking alien, and the Doctor has read Douglas Adams in the past.
A monstrous Eldritch Abomination so powerful it can be considered a deity even by someone like the Doctor, who for the good of the universe is kept dormant by way of endless singing, and should the aforementioned music cease for a second it will wake and destroy everything... uh yeah, this story literally has the Doctor directly confronting Azathoth. Which makes two Outer Gods in a row; The Great Intelligence is Yog-Sothoth in-canon (Expanded Universe only).
The Unintelligible: Most of the aliens in the bazaar speak in gibberish, including one that speaks in barks. This despite the fact a well-established convention in the series is that the TARDIS is supposed to translate alien languages for the Doctor, the companions and us, the viewers (though it is established that the TARDIS doesn't like Clara, so maybe she decided not to translate everything, just to be a pain).
Visible Boom Mic: The second time Clara encounters Merry (when they startle each other during their game of cat and mouse), though the sound guy does notice quick enough to lift it back up fast enough that it becomes a blink-and-you'll-miss-it moment. Interestingly, all shots focused on Merry in that scene following the hiccup are close-ups.
When the Planets Align: When the Rings of Akhaten align every thousand years or so, an extremely important festival is thrown. Well, important locally. He compares the whole thing to "Pancake Tuesday".