When Finn is saved from the Time Rangers by an older version of himself, it comes with a catch. Old Man Finn needs Finn and his friends, Lincoln and Julep, to help him win battles he lost years ago against the Paradox, a terrible creatures obsessed with revenge against Finn's father. If they succeed, he promises to tell Finn where his father is. He even gives them a pair of time-traveling pajamas to help them with their quest.

There's only one problem. The pajamas malfunction almost immediately. Along the way the kids avoid being eaten by sabretooths, shot by revolutionary war soldiers, and start a war with a town full of holograms, all in an effort to fix the jammies.

Just when their goal is in sight, Julep gets sick. Only...it doesn't seem like she's sick. It seems like she's changing. The adventure may cost Finn more than an old pair of pajamas--it may cost him his friends!