Lindy's mother married Nathan's dad when the two were in third grade. Since then, they've been as close as any real siblings would. Nathan and Lindy are very much alike, sharing many things in common, including a shared lack of intelligence.
Known for being the stupidest kids in their school, Nathan and Lindy are mocked and belittled by teachers and classmates. After the two flunk their math test, they ask their Uncle, Dr. Frank King, for help in boosting their intelligence. As a solution, he gives them a purple liquid that he calls 'Brain Juice'. Unbeknownst to the pair, the liquid is just normal grape juice, which their Uncle hopes will be enough to boost their self-confidence.
At first, Nathan and Lindy are disappointed to find that nothing has changed. However, they eventually get their wish to be intelligent when two aliens named Morggul and Gobbul replace the regular grape juice with Brain Energizer Fluid from their home planet. Soon, the step-siblings become geniuses. They start passing every test, correcting every mistake their teachers make. While their newfound intelligence is at first a blessing, it becomes a curse when they are slowly shunned by their peers and friends. Eventually, their school decides to kick the two out for being "too smart."
They are later approached by Morggul and Gobbul, who reveal what they did. The aliens tell them that they needed a pair of earthlings to take back to their planet, in order to serve as slaves for the emperor. Making them intelligent was the first step in their plan. The two aliens abduct Nathan and Lindy, presenting them before their emperor.
However, he is displeased to find the pair aren't as intelligent as Morggul and Gobbul said. Their genius has been reverted back, and the emperor disintegrates Morggul and Gobbul for their failure. Before he can kill them too, Nathan and Lindy escape back to Earth.
It is revealed at the end of the book that the two faked their stupidity and have still retained their genius intellect. However, the two agree they must keep this a secret.
Nathan and Lindy start off the book as unintelligent. However, they are still kind and get along well as a pair of siblings. When their intellect grows, so does their ego. When the spotlight is put on the two for being geniuses, they seem afraid, and dislike the attention, especially since it has cost them their friends.
Nathan is described as having dark eyes, black-rimmed glasses, and curly black hair. He looks older than Lindy, despite them both being twelve.
Lindy is described as tall and thin, with long, straight auburn hair and green eyes.