With Simeon now at every point of the Doctor's timeline, we see various old Doctors encountering him and going wrong, all being killed. The Dalek Asylum; London. Vastra heads outside to survey the damage among the stars. As she explains, "A universe without the Doctor. There will be consequences."
And yes: One by one, so many local star systems are being lost, all the lives saved by the Doctor in his lifetime being un-saved. Even Jenny. Even Strax, who attacks Vastra before disappearing (dissolving?).
As the Doctor writhes, Clara is just realizing how impossible she really is, how she has been able to die and die and die and still save the Doctor. She has to do it again -- for the first time. She has to enter his time stream. River warns her that the time winds will tear Clara into a million pieces, all living and dying, but none truly her. She will become copies. So be it: Like Clara's mom always said, the soufflé isn't the soufflé, it's the recipe. (Clara is Soufflé Girl after all.) With a final line -- "Run, run you clever boy...and remember me" -- Clara leaps into the time stream. The episode's beginning voice-over replays. I realize now that it isn't an abyss that Clara is falling into, it's time. She saves each Doctor, and even tells the First one which TARDIS to steal.
The Doctor's time stream changes, from its wounded red to blue again. They are restored, Strax and Jenny and Vastra. But not Clara. The Doctor insists on saving her, though River warns him that stepping into his own time stream is madness. Use the TARDIS, use something, but be sensible! This she says with a slap, but he catches her slap -- he can see her, and hear her, too."You're always here to me," he tells her. This is both good news and bad: River is just an echo now, and she should be fading. But she can't, not without a proper goodbye. But the Doctor can't, he doesn't know how. (So sad, y'all.) The only way is the best way, River says, to leave her as if he will return. He works his face a bit and smiles. "See you around, Professor Song."
One more thing, before the Doctor jumps in: River is still mentally linked to Clara, which means she's still alive. How? River smiles before fading away. "Spoilers."
NEXT: The Doctor's secret