From the book:

Most of the fourth graders signed up for the National Student Exchange Program. Your exchange buddy was a kid from another state. He or she would come to live at your house for a month, and go to school with you as a guest student.
Joey Petrillo already had a great exchange buddy, Cody. Cody was from Texas. He lived on a real ranch. He could rope and tie a calf in less than ten seconds. And Calista Bernstein's buddy, Wanda, came from California. She dressed like a super model and even wore make-up!
Come to think of it, all the exchange buddies had turned out to be pretty cool. That's why I was so excited about meeting Stan. Was he a star quarterback? A Nintendo master? A laugh-a-minute class clown?
Mom checked her watch. "Where could he be?"
Good question. All the passengers were off flight 407. They were picking up their suitcases at the baggage claim.
Suddenly the luggage carousel groaned to a halt. The lights flickered once, and then the whole airport went dark.
"Stay close to me," ordered mom in the gloom. "It's a power failure."
The words were barely out of her mouth when the lights came on again. The carousel sprang back to life and more luggage came down the slide. Out rolled a steamer trunk, a garment bag, two duffles, and a person! He tumbled down the chute, hit the carousel, and started circling like he was a suitcase.
I was so shocked that I broke Rule 6.
"Mom, did you see that?"
But she was looking the other way.
A shiny silver travel bag came down and bonked the guy on the head. He picked it up, jumped off the carousel, and landed at our feet.
What a nerd this kid was! He had a crew-cut, and big thick glasses that made his eyes look like fried eggs. Worst of all, he was wearing a tie! Didn't he know about Rule 21: No ties? His pants were too short! His sleeves were too long! I think his belt was up under his arms somewhere."Beat it, kid," I whispered. "I'm looking for my exchange buddy!"
"You must be Devon Hunter," said the nerd with a little munchkin voice.
"Oh no," I prayed to myself. "Please don't be Stan."
He nodded, grinning. Even his teeth were nerdy. "I'm Stan."

Copyright © 1999 Gordon Korman used by permission

Devon's main goals in life were to be cool, and to enjoy spending time with his exchange buddy. So, boy, was he in for a shock when his exchange buddy turned out to be the least cool person he'd ever met. Stan wore ties, talked like a dictionary, preferred paper to food for his daily diet, and talked to the family dog (in Dog.) Worse, he would pick his nose, even in class. Devon soon found himself and his exchange buddy to be the butt of all the jokes at school.

But as Devon started learning more about his 'buddy,' the kid went from nerdy to strange. He washed and dried all the dishes after dinner in under a minute, and any time he found himself face to face with a mirror, he went cross-eyed and started talking gibberish.

When rumors of flying saucers start flying around town, Devon begins to suspect something is really creepy about his new 'friend,' but as he starts to learn that Stan might be Earth's only hope of salvation, he realizes it is up to him to help the ultimate nerd fit in!