“But when it comes to the river of history, the watershed moments most susceptible to change are assassinations — the ones that succeeded and the ones that failed. Archduke Franz Ferdinand of Austria gets shot by a mentally unstable pipsqueak named Gavrilo Princip and there’s your kickoff to World War I. On the other hand, after Claus von Stauffenberg failed to kill Hitler in 1944—close, but no cigar — the war continued and millions more died.”

I had seen that movie, too.

Al said, “There’s nothing we can do about Archduke Ferdinand or Adolf Hitler. They’re out of our reach.”

I thought of accusing him of making pronounal assumptions and kept my mouth shut. I felt a little like a man reading a very grim book. A Thomas Hardy novel, say. You know how it’s going to end, but instead of spoiling things, that somehow increases your fascination. It’s like watching a kid run his electric train faster and faster and waiting for it to derail on one of the curves.

“As for 9/11, if you wanted to fix that one, you’d have to wait around for forty-three years. You’d be pushing eighty, if you made it at all.”

Now the lone-star flag the gnome had been holding made sense. It was a souvenir of Al’s last jaunt into the past. “You couldn’t even make it to ’63, could you?”

To this he didn’t reply, just watched me. His eyes, which had looked rheumy and vague when he let me into the diner that afternoon, now looked bright. Almost young.

“Because that’s what you’re talking about, right? Dallas in 1963?”

“That’s right,” he said. “I had to opt out. But you’re not sick, buddy. You’re healthy and in the prime of life. You can go back, and you can stop it.”

He leaned forward, his eyes not just bright; they were blazing.

“You can change history, Jake. Do you understand that? John Kennedy can live.”