The 12-year-old protagonist, Regan, attends a school where students learn to time travel, training for careers as “Glitchers.” Tampering with the past is illegal because it can cause a chain of events that causes problems in the future. Regan and her classmates are learning to enter the past so they can catch time-traveling criminals called “Butterflies,” who seek to change history. Regan hasn’t started time-traveling yet; instead, she and her classmates practice in simulations.

Currently, Regan is in a simulation of the night when Abraham Lincoln was assassinated by John Wilkes Booth in a theater. She’s supposed to apprehend the Butterfly who seeks to prevent the assassination, which Regan feels conflicted about. Even though protecting the president seems like a morally good idea, experts argue that this could cause a disastrous chain of events, so Lincoln unfortunately needs to die. Butterflies often want to impede social progress, so it’s not a difficult decision to stop them then, but this simulation is different and is meant to teach the students that sometimes, they need to put personal morality aside and focus on preserving history.

Regan must apprehend the Butterfly before the assassination without accidentally causing any other changes to history.