Forget reappearances from the Master, the Cybermen and Captain Jack – the latest episode of Doctor Who opted for a more deep-cut reference to the long-running show's past.
Can You Hear Me? – by showrunner Chris Chibnall and a writer new to the series Charlene James – saw the Doctor (Jodie Whittaker) and friends lured into a trap by Zellin (Ian Gelder), an immortal "god" who was haunting the dreams of humans, all to feed his beloved Rakaya (Clare-Hope Ashitey).
The Doctor remarks that 'Zellin' is "a mythical name, [from] way beyond this universe", with Gelder's villain confirming that he and Rakaya are both all-powerful, ever-living beings, pitting two planets against each other purely to pass the time.
“We immortals need our games, Doctor," Zellin says. "Eternity is long and we are cursed to see it all.”
He continued: "The Eternals have their games, the Guardians have their power struggles. For me this dimension is a beautiful board for a game... the Toymaker would approve."
Confused? If those references to Doctor Who history are lost on you, here's a handy explainer...
Making their debut in the 1983 story Enlightenment, the Eternals are a race of elemental beings of immense power, capable of manipulating matter and creating objects out of thin air.
These amoral creatures, like Zellin, act purely for their own amusement, manipulating "Ephemerals" (read: mortal beings) for fun.
Though their origins are uncertain, the Eternals are said to live 'outside of time, in the realm of eternity' and, in their first Doctor Who appearance, they participated in a race through space arranged by the Guardians of Time (more on them below).
The prize would be "enlightenment" – the granting of their heart's desire brought to life, symbolised by a crystal. The Eternals copied ships from Earth history, fitted them with "ion drives" and sails to catch solar winds and used kidnapped humans to crew the crafts.
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Though Eternals cannot be destroyed, the fifth Doctor (Peter Davison) and his companion Turlough (Mark Strickson) were able to defeat Captain Wrack (Lynda Baron), an Eternal in league with the Black Guardian, by throwing her overboard into space.
Though they didn't appear in Doctor Who again, the tenth Doctor (David Tennant) referenced the Eternals in 2006's Army of Ghosts, while the witch-like aliens the Carrionites mentioned how the Eternals had banished them "into deep darkness" soon after the "dawn of the universe" in 2007's The Shakespeare Code.
The aforementioned Guardians first appeared in Doctor Who's 16th season in 1978, a series of interlinked stories which saw the fourth Doctor (Tom Baker) on a quest to find the legendary Key to Time.
Transcendental beings who embodied aspects of the universe, immortal and indestructible, we met the White Guardian (Cyril Luckham) – who represented light, order and structure – and his eternal opponent the Black Guardian (Valentine Dyall) – the personification of darkness, entropy and chaos.
The White Guardian sent the Doctor and his companion Romana on a mission to find the Key to Time, its pieces disguised and scattered across the universe, warning them that the Black Guardian planned to use the key as a weapon. Though the Doctor was able to reassemble the key, he eventually scattered the pieces back through time in order to prevent it falling into the Black Guardian's clutches.
The Black Guardian sought revenge five years later (our time), recruiting the exiled alien Turlough to kill the Doctor in 1983's Mawdryn Undead. Turlough began travelling with the Time Lord and grew fond of him, turning against his master. He threw the enlightenment crystal at the Black Guardian, who vanished in a burst of flame, though the White Guardian warned that his nemesis could never be truly killed.
So how does the Toymaker fit into all this? Well, retroactively...
Before Doctor Who canon was all that convoluted, the first Doctor (William Hartnell) encountered the Celestial Toymaker (Michael Gough) in a 1966 story bearing the villain's name.
Here, the Doctor and his companions arrive in an otherworldly domain overseen by the Toymaker – another immortal entity, who forces them to play a series of games, with the outcome deciding whether they will remain his playthings for all eternity.
The Doctor was of course able to outwit the Toymaker and escape, with the character never making an encore – plans for a comeback in the 1980s fell by the wayside when Doctor Who was put on hiatus by the BBC during the Colin Baker era.
The Toymaker's origins were never explained – then-Doctor Who script editor Donald Tosh later revealed that, years before the Time Lords were established in canon, the Toymaker was supposed to be a member of the Doctor's own race.
2001 spin-off novel The Quantum Archangel suggests that he is instead another Guardian a la White and Black – the Crystal Guardian. This has never been verified on television, though Zellin's comments in Can You Hear Me? do appear to confirm a link of some kind between all three beings...
Doctor Who continues on BBC One at 7:10pm on Sundays