Always the new kid
Twelve-year-old Julian Newcomber knows he has a fitting--what the smart folks call apropos--last name. His family moves constantly due to his Dad's quirky inventions, which have a habit of misbehaving and blowing up the family home. So he's always been the new kid in school.
Because of his father's inventions
Now firmly settled in a peaceful and close-knit community, Julian believes he's finally found a home, not to mention a safe haven from his father's calamities. That is, until a vaguely familiar 20-year-old man walks out of the bedroom closet, and identifies himself as future Julian.
Until his father goes too far
It turns out that his father's latest contraption, the eTab, alters time. So Grown-Up Julian decided to take a joyride into the past and dropped his cell phone on a Civil War battlefield. (Who wouldn't want to see Pickett's Charge, after all?)
And his future self needs his help
With his own eTab too low on juice to take him all the way back to his own time, Grown-Up Julian needs Young Julian to locate a quantity of Cucumium so he can go back to the Civil War and retrieve his cell phone, then get home before his parents find out.
Before history is rewritten
The two Julians break rules, break curfews, and break into the Intermediate School in an effort to locate some Cucumium (though they're not exactly sure what it is), so they won't be forced to share the room and fight over the top bunk forever. Oh, and if Grown-Up Julian stays, it could, as a side effect, pollute the natural timeline and rewrite history as they know it.
No pressure.