Keanu Reeves doesn’t know exactly where the idea came from, but one day he imagined a man who couldn’t die. Now, he’s releasing his debut novel, “The Book of Elsewhere,” which he co-wrote with the British science fiction author China Miéville.

Keanu Reeves doesn’t know exactly where the idea came from, but one day — sometime around the release of “John Wick: Chapter 2,” starring Keanu Reeves, and before he started shooting “The Matrix Resurrections,” also starring Keanu Reeves — he imagined a man who couldn’t die.

“It became a series of what ifs,” he said. “What if they were 80,000 years old? Where did this character come from? What if they came from a tribe that was being attacked by other tribes and wanted to ask the gods for a weapon, and what if a god replied, and what if that birthed a half-human, half-god child?”

From there, Reeves added, “It went from this simple premise and gained in complexity and continued to grow.”

For a while, the character only existed in Reeves’s head. Then he wondered, What if this immortal warrior became the basis for a comic book? An action movie? An animated series?

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“And then, there’s another what-if,” he said. “What if it became a novel?”

Reeves’s ancient warrior has since become the anchor of a growing multimedia franchise. The comic he imagined and co-wrote, BRZRKR (pronounced “berserker”), grew into a 12-issue series that has sold more than two million copies. A live-action film, starring and produced by Reeves, and an animated spinoff are in development at Netflix.