Julia's fifth grade class has developed the strange problem of suddenly disappearing, then reappearing minutes later as if nothing had happened. Julia tries to find out what's up with that, and enlists the help of her easily convinced friends Lori and Jeff, and starts investigating. This is even before Julia proves it to them by switching their backpacks and turning a painting upside down while the class is gone before they reappear.

Julia's investigations take her inside a nearby military base that is literally right next door to the school, where she discovers the Vanisher, a machine that was being designed to make things disappear and reappear. She accidentally messes it up and gets herself embedded in a wall, half of her on one side, half on the other, freaking out a guard, but is soon freed by the scientist who built the device.
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!This book contains examples of:
* ChildHater - The principal's secretary, nicknamed "The Dragon Lady" by the kids. While she's particularly nasty to kids, even other adults seem to dislike her.
* TheGuardsMustBeCrazy - Arguing about sports teams and blurting out the password to a door ("It's easy to remember this week because it's my birthday 5, 14, 52.") make for a military base even a kid could break into. Or three.
* HeKnowsTooMuch - The military brass don't want to let Julia, Lori and Jeff leave because the kids know their secret. Julia even speculates that they might be killed.
* SkepticismFailure - Paraphrased here: "Did you know that our entire class just disappeared, including you guys?" "Really? I didn't feel anything." "Yeah, I saw it and even left the room and asked for help." "Let's investigate!"
* SymbolSwearing and NarrativeProfanityFilter - Both occur in a row.
-->''"Oh, my, #$@#$." He said a bunch of words my mom would never let me use.''
* TakeThat - To ''MyTeacherIsAnAlien'', which was released a few years prior. That book had a scene in which the young hero followed a teacher all the way to his house, walking several blocks. In this book, however, Julia tries to follow the teacher home... but the teacher gets into a car. Julia curses herself for wasting her time and wonders, "Why didn't I think she'd drive a car? Just because kids walk everywhere doesn't mean adults do!"
* {{Technobabble}} - [[LampshadeHanging Lampshaded]] when the scientist is asked by a military official, "I suppose you have some scientific babble to explain this?" Then played semi-straight as the scientist tries to explain how his Vanisher works, but the narrative only sums up with some of the "big words" he uses and doesn't show his entire dialog.
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