On his twenty-fifth birthday, Diesel Lagerford meets “Twenty-Six,” a one-year-older version of himself. Twenty-Six tells Diesel that there are others like them in parallel timelines spanning the ages of twenty-five to sixty, and they can visit each other’s worlds using a machine called a T-box. He introduces Diesel to Lilah Spencer, the AI developer who maintains the T-box, and Diesel is smitten. But when he travels up the timeline, Diesel learns that Lilah will soon die under suspicious circumstances. Devastated, he and Lilah agree that a super-AI could solve the mystery of her death, and they race to create an artificial intelligence of extraordinary capability. But can they finish their work in time to save Lilah? And will the super-being help them in the ways they had hoped?