Sprinting across a wartorn planet, Bill Potts reflects on the strange turn that her life has taken ever since she met the Doctor. As he pauses to catch his breath, Bill's attention is drawn to a statue of the Ninth Doctor. Dragging Bill into the temple, the Doctor does the Y.M.C.A. dance to open a secret door which opens to reveal the final resting place of the Doctor's old friend Plex, the loneliest creature in the universe. Opening the tomb, the Doctor pulls out a shattered biodata module and realises that he'll have to do this the old-fashioned way: by telling them a story. After having Bill draw the natives' attention, the Doctor starts his tale. Soon afterwards, the two are back in the TARDIS with Bill asking for the truth instead of the myth he just told.
It wasn't long since the Doctor had regenerated and, still on the run from his last face, paid a visit to his friend Plex to inform him that his planet had been lost to a freak singularity. Rising from his bed, Plex guides the Doctor to a store of artron energy that he siphoned off from the TARDIS the last time the two met. Plex hopes to use the energy to fuel the creation of a race of clones grown in his own image, though modified just enough to allow for a viable gene pool. After the Doctor expresses his sympathies, he gifts Plex the biodata module that he promised.
A few years later, the Tenth Doctor, joined by Cindy and Gabby, came to pay his respects to the departed Plex before a pre-recorded hologram of the alien pops up. Plex reveals that copying the clones' neurology after his own has resulted in passive race in perpetual agreement. So for his last request, he asks the Doctor to make the clones forget their origins so that they will be able to disagree and move forward.
Unfortunately, it didn't quite work out as the Doctor intended. Coming back with Alice and the Sapling when he had "the chin", the Eleventh Doctor found a feudal world at war. Running to Plex's tomb, the Doctor grabs the biodata module and uses some regeneration energy to bring Plex back to life in one of the clones. After thanking his friend and assuring him that such conflicts are inevitable in the long road of societal reconstruction, Plex makes peace among his people before the Doctor returns the module to the tomb, noting that he would hate to have to be the one who did all the explaining.
Finishing his tale, the Doctor notes that it's entirely possible that the clones will go back to killing each other. But in his long life, while he's learned not to let such disappointments get him down, it's the surprises that always takes his breath away.
The Tenth Doctor uses a psychic link, maintained through touching an individual's temples, to wipe their memories. He'd previously done this to Donna Noble, under dire circumstances, (TV: Journey's End) and would later almost do the same to Bill. (TV: The Pilot)
The Twelfth Doctor remembers his previous incarnation's chin with fondness, while his tenth incarnation previously mocked it. (TV: The Day of the Doctor) The Twelfth Doctor has shown an open approval of his previous appearance before. (TV: The Caretaker)
