The Doctor pressed his hands against Sherlock's forehead. "I'm sorry, my boy, so sorry. But I have to—"
"I know you do," he grunted. "I know everything you do, remember. Pity, though, my notes on regeneration would revolutionize—Ah!" His muscles sagged, but the Doctor caught him half-way down.
"Doctor, what happened?" Iris whispered. "I don't understand."
The Doctor set Sherlock on the floor, loosening the boy's scarf so he could breathe more easily. "Human minds are like attics or cellars, they only have so much space in them. That regeneration energy gave him some of my memories. Normally he could cope—it wasn't that much, but with Sherlock—" he paused. "He notices everything. He's seen stars and satellites and Time itself at the Untempered Schism. So I had to erase some memories, to make room."
"Which ones?"
"I was just trying to take out the ones that were mine, the ones he wasn't supposed to have." The Doctor stared at the floor. "But they were too interwoven, I didn't have time to separate them. I removed them, but also his memories of us."
"Us?" Iris shivers. "No. No…please, no."
"I'm sorry, Iris." The Doctor holds her for a moment. "But he's going to have to leave."