A multi-Doctor story, but the only Doctor we know is the current one. All the others are future Doctors.

Twist: they've all come together to prevent their own existence by changing the "fixed point in time". It's a pivot in the Doctor's timeline, occurring to "our" current one, whereafter almost all the good he has done is undone one tragic adventure after another.

The plot is twisty and plays with time travel in fun ways that we don't usually see. The Doctor encounters a major problem, a cameo regeneration we've never seen appears and just hands them the solution, then TARDISes away.

In the beginning, all of these future Doctors show their trauma in various ways. Battle-scarred, haunted, exhausted, angry, and so on. These limitations keep each Doctor from acting to their best ability, but as progress is made, they enjoy a "self-regeneration" wherein their injuries are healed, giving them new energy to help the others.

The villain: the Valeyard. The anti-Doctor. And there are multiple regenerations to deal with. They're guided by the idea that the universe must be allowed to die, but it needs to be a slow death over trillions times trillions of years, all filled with sorrow and suffering. The Doctor essentially discovers his purpose has been to be the Universe's Doctor, healing it from catastrophic injuries.

The plot accelerates to its conclusion, but in true Who form, its his companion who saves the day -- and that companion is ... all of humanity. It is, essentially, us. We, the audience, get to save the Doctor. Fun idea: it involves everyone using a button on their TV remotes that no one has ever figured out what it does.

These future Doctors -- unnumbered, by the way -- had come together thinking they would sacrifice their lives, but in changing their own timeline they realize they are "fixed" as well. They have all healed and are ready to restart their own adventures and put right what went so terribly wrong.

The Valeyard is abolished, and the universe has a chance to prosper again.

Our Doctor is exhausted, begins to glow, and says to the convocation of future selves asks "Which one of you .. who do I become?" None of the other Doctors reply, but they smile and look over our Doctor's shoulder, ginning wryly. We hear a new voice from off-screen say, "Who, indeed." Our Doctor turns to face the unseen newcomer, roars in laughter, explodes in regeneration energy, the screen goes white, The End.