From Tardis Wiki, the free Doctor Who reference

Superficially, the Prometheans were a very elegant race, possessing smooth gold skin and long-limbed bodies, identifying themselves as teachers and shapers out to aid the progress of lesser races, claiming to have done so since the dawn of conscious thought. However, their appearance concealed a cold heart, disdaining emotions among "lower" races — dismissing the Doctor as arrogant and undisciplined — claiming that they were doing humanity a favour by stripping them of their higher emotions, when their true objective was simply to reduce human mental development to a point where they could do nothing but worship the Prometheans, regarding their actions as eliminating the higher concepts that troubled humanity so that they would no longer be troubled by disease, injury, or environmental changes.

The Prometheans were attracted to Earth when the First Doctor materialised in the distant past just as they were passing by, the chronal ripples created by the TARDIS's materialisation drawing them to Earth in time to witness how the First Doctor, Susan Foreman, Ian Chesterton and Barbara Wright — particularly Ian and Barbara — taught the primitive Tribe of Gum how to make fire, unite against others in defence, and the strategic benefits of subterfuge. Feeling that the TARDIS crew's work to inspire the Tribe of Gum to develop should continue after their abrupt departure, the Prometheans supplied the tribe with various samples of psychic metal, which the Tribe shaped into weapons as they became leaders of the other tribes. However, when their immediate supply of psychic metal was exhausted as the tribe grew too large, the Tribe of Gum were sent out to find more metal, equipped with armour that would slow their ageing and allow them to withstand even faster-than-light speed without any trouble, but also asked that the Prometheans provide them with "Teacher" and "Woman" — Ian Chesterton and Barbara Wright — so that the Tribe could learn more from them.

Over the centuries, as the Tribe of Gum searched space for more psychic metal — their efforts warping space so that they only aged a few years as they travelled despite the subsequent centuries — various other samples of psychic metal were found on Earth as humanity developed. These samples included an old woman in ancient Greece using a "Belief Engine" powered by a piece of the metal to create the gods of Olympus, an alien artist imprinting his name on the entire population of London — turning everyone in the city into a silhouette figure with his name "written" on them — and a sixteenth-century Czech rabbi transferring his mind into a golem powered by an orb of psychic metal. Although the Eleventh Doctor, Amy Pond and Rory Williams were able to resolve these problems, the Doctor was left concerned about both the origin of the psychic metal and a mysterious message that those who used the metal asked him in the aftermath: "What lies buried in man?"

The Doctor was drawn back into the search for the psychic metal when he encountered a team of temporal mercenaries leaving Earth in 1965. Having knocked out most of the crew, the Doctor discovered that the ship contained a cell made out of psychic metal resembling the Pandorica. He entered the cell and discovered that it held Ian and Barbara, captured just as they returned to Earth, the cell having erased their memories of their time with the Doctor. The Doctor was able to remind them of their travels by mentioning Susan and drawing a sketch of the TARDIS on the blackboard, but it took a while to convince Ian and Barbara of his identity due to the changes both he and the TARDIS had undergone since they travelled together. While investigating the planet where the bounty had been sent, the TARDIS crew witnessed the Tribe of Gum attacking the planet to find another sample of psychic metal, Ian managing to defeat one of the Tribe with a sword of psychic metal that the Doctor had acquired in a previous trip. Although Barbara was taken prisoner by the mysterious 'Miss Ghost' - later revealed to be an agent of an anti-alien branch of MI5 using salvaged technology - the Doctor and Ian tried to escape, only to encounter the Prometheans, who revealed the true identities of the Tribe of Gum and explained what they had done to them.

As he and Ian explored an illusion the Prometheans had trapped them in, the Doctor learned that they had been using the psychic metal over the centuries to 'encode' the image of the sun as a symbol of worship into humanity's subconscious race memory. As The Doctor confronted the Prometheans' claims to protect humanity by pointing out that they were simply preserving their specimens from outside influence, the Prometheans attempted to break the Doctor by forcing him to relive his memories of the destruction of Gallifrey and other losses. However, while the Doctor was briefly ashamed at the reminder of the people he had killed in his lives, Ian encouraged him to focus on the examples of his courage and willingness to die for others instead, each remembering what the other had taught them, such as Ian learning courage from the Doctor, while the Doctor learned compassion from Ian and Barbara that inspired his later interest in companions.

When the Doctor and Ian returned to reality, Barbara and Miss Ghost found the Prometheans' vessel, a massive structure of psychic metal, twelve thousand miles in diameter, in the shape of a sun. Barbara convinced the Tribe of Gum to let them join the Doctor and Ian, just as the Prometheans revealed their true plan. Having spent millennia subtly encoding the image of the sun as an object of worship into humanity's subconscious, the Prometheans triggered the psychic metal, regressing humanity back to a Stone Age mentality where they would venerate the Prometheans and follow the Tribe of Gum as the Tribe provided them with vital resources. Although destroying the structure would not undo the transformation, a comment by Ian gave the Doctor an idea. Having taken a quick detour into his past to break the TARDIS's chameleon circuit in Foreman's Yard to ensure that it remained in the form of a police box, the Doctor returned to confront the Prometheans, revealing that, while they had encoded the image of the sun into the human psyche, his travels throughout human history had encoded a second image in humanity's subconscious; the symbol of the blue box as a symbol of protection and strength (Revealing that The Doctor himself was the answer to the question 'What lies buried in man?'). While the Prometheans still had access to the human subconscious, the image of the TARDIS - the Doctor having reshaped the Prometheans' vessel using the gauntlet of psychic mental that Monos had used - would prevent them from enforcing their will on humanity, the box-image reminding humanity not to fear the dark.

Although the Tribe of Gum attempted to attack the Doctor for depriving them of leadership of the Many Tribes of Earth, Ian and Barbara confronted the Tribe with the knowledge that they were still learning about the power of love and compassion, explaining that the Doctor had helped the Many Tribes of Earth for so long because he loved them and belonged to them just as they belonged to him. Outraged at the destruction of their plans, the Prometheans attacked Ian and Barbara, inspiring the Tribe to turn on the Prometheans, both sides wiping each other out as the Prometheans' power was matched by the Tribe's rage. (COMIC: Hunters of the Burning Stone)