There was some talk about "interrupting the timeline" which, honestly, went a bit over my head. Here's what I got: The creatures came into the Eye of Harmony; the Not-Android tackled them, and they fell into the abyss below; the Brainy Brother stayed behind to pull his brother up -- stayed just long enough, alas, for their molecules to fuse together. They became the weird two-headed creature -- the same creature who had been hunting them. The paradox was obvious, and I suppose it only really worked because the timeline was locked inside of the TARDIS.
This was an intriguing twist, although -- in all honesty -- it feels like there were many, many more possibilities left unexplored inside of the TARDIS. Weirdly, the basic idea of exploring the TARDIS almost seems like it could've been the 50th Anniversary Special -- which would have also given the Doctor the opportunity to interact with his own past, not to mention his future.
In any case: the Doctor and Clara ran into the center of the TARDIS -- which revealed itself to be a gigantic cliff face. There was nowhere to go. And, the Doctor thought, no use mincing words anymore. "Tell me who are you," he told Clara. "Why do I keep running into you?" He ranted at her, a pent-up declaration that made no sense to her: Meeting her in the Dalek asylum, meeting her again in Victorian London, her two deaths. "What are you? A trick? A trap?" Clara was terrified. She had nothing to tell him. She was just Clara.
(ASIDE FOR THEORY TIME: This is the third time this season that the Doctor has directly addressed the mystery of Clara -- and this is the third time this season that he got nowhere. He tracked Clara's life all the way back to her parents' meeting: Normal. He asked last week's empath what she saw in Clara -- and the empathy just saw a normal girl. Now, he asked Clara point-blank, and she couldn't tell him anything. Are we being set up for a big twist in the next few episodes? I wonder if, in a weird way that will only become clear later, the creators are using the many lives of Clara as a way to explore the dark side of the Doctor's Companions. The first two times he met Clara, she wound up dead -- as if the Doctor's mere appearance was enough to bring Death behind him. Will Clara die a third time? END OF THEORY ASIDE.)
Clara talked the Doctor down. He was calm. He looked in Clara's eyes, and saw nothing but Clara. He realized what they had to do -- they had to jump. And they landed in the Heart of the TARDIS. (NOTE: This marks yet another "Heart" reference, continuing a trend we've seen all season.) Problem: The engine had already exploded, and the TARDIS had frozen it in the moment of explosion. But there was nothing the Doctor could do to save her anymore. "She's always been there for me, taken care of me," he lamented. But he couldn't take care of her.
NEXT: "You have a name."