The cover artwork shows a brain with strange, purple liquid being poured onto it.
Nathan and Lindy Nichols are step-siblings with a reputation for being the stupidest kids in school. While visiting their uncle Dr. Frank King, they ask if he has anything that will help them become smarter. He fools them into drinking normal grape juice to get them off his back. This actually turns out to be 'Brain Juice'. Soon however, the two become the smartest kids in school -- so much so that their principal, Mrs. Lopez, has them kicked out for being too smart.
Surrounding their expulsion from school, Nathan and Lindy are soon hounded by news anchors who are reporting the controversy over the two being kicked out of school, agents who want to make them famous, and government scientists who want them for experiments. After a while, Nathan and Lindy get so terrified at the thought of being famous.
As Nathan and Lindy run away from home, two aliens, Morggul and Gobbul, finally catch up with them and reveal that they planted "Brain Juice" in the grape juice at their uncle's lab so they can take back two Earthlings to be slaves to the emperor of their home planet. Nathan and Lindy are abducted and taken to Gorbbl and Morggul's home planet to be presented to the emperor, but the effects of the Brain Juice begin to wear off and Nathan and Lindy soon act stupid again. Gobbul and Morggul are disintegrated, while Nathan and Lindy are sent back to Earth. In a twist ending, it's revealed that Nathan and Lindy were only pretending to be stupid so they won't be slaves to the alien emperor and must keep pretending to be stupid in order to return to their normal lives. Dr. Frank drinks to the children's tale, but Nathan asks him what he's drinking. He says it's the same grape juice he gave them, which he's been drinking eight times a day.
- Unlike most books in the series, this book doesn't start with chapter one. Instead, it starts with a prologue.
- In an interview with Entertainment Weekly, R.L. Stine said that he really liked this book (in fact, it is one of his favorites), but it was always overlooked.
Reference in other Goosebumps media
- This book is referenced in "Goosebumps: The Game" as an achievement called "Using Good Ol' Brain Juice".