The book follows Larry Boyd. He and his friends find a bottle of expired tanning lotion in the garage and rub some on. Soon after, Larry begins sprouting hair on his hands, his friends keep disappearing, and there are more dogs in the neighborhood. Is the tanning lotion to blame?
The cover illustration features a boy Larry looking at the mirror in the bathroom, noticing his arms and parts of his face covered with blackish hair.
Larry Boyd is a boy with wavy blonde hair and big ears. He is running from a pack of stray dogs. The strays are all over his town. Larry slides across ice and snow, as he races towards his friend Lily's house. Lily (a girl with blonde hair and two different eye colors) takes a snow shovel and she waves it around in a threatening manner. The dogs flee. Larry was going to Lily's house, so that they and their friends Manny (a tall and skinny boy), Kristina (a chubby girl), and Jared (who is always wearing a Raiders cap) can rehearse for the upcoming Middle School Battle of the Bands. Their main competition is a band called Howie and the Shouters, the superior group led by a jerk.
The children decide to take ten and go play in the snow. While digging through Lily's neighbor's dumpster, Larry comes across a discarded medicine cabinet. He retrieves a bottle of tanning lotion by the name of INSTA-TAN, RUB ON A DARK SUNTAN IN MINUTES. All five members of the band think that it would be really awesome to get a fake tan during the winter. They excitedly race into the bathroom and take turns squirting the liquid on their bodies. Larry initially refuses, as he notes that the lotion expired several years ago. Larry expresses concern, but nobody listens to him. Then Larry is pressured to apply the lotion when the group asks, what harm an expired tanning lotion could really do.
The group participates in a snowball fight, in which Larry faints and collapses. Dr. Murkin, Larry's pediatrician, gives Larry his regular bi-monthly shot and reminds the boy that he is not supposed to overexert himself, as he does not have sweat glands. After his injection, Larry races back to the snowball party. There, he discovers that Howie came by to gloat about having bought an Eric Clapton songbook for the contest. That night, Larry is simply exhausted. But before he can drift off to sleep, he is shocked to discover a thick patch of thick, bristly, oily hair growing on his palms. He races to the bathroom and frantically shaves his palms.
The next morning, Larry is paranoid about his hair growth. Everyone already calls him "Hairy Larry" because of how much he loves his blonde hair. He tries asking Lily if she experienced the same thing as Larry assumes that the growth is the result of the INSTA TAN. In class, Larry goes up to give his Bruce Coville report, but Howie trips him. While picking himself up off the ground, Larry notices the hair has grown back around his hands, this time covering both palms. He runs out into the hall and retrieves his gloves from his locker.
Larry shaves off the hair again, covering his hand in cuts, and becomes worried that it will keep growing back and possibly replace his beautiful hair. A paranoid Larry, worried about what is happening to his body, soon discovers hair growth on the knees and legs. Sad to say, his yellow-eyed cat Jasper wants nothing to do with him. Larry refuses to wear shorts to gym class and temporarily avoids a crisis. Larry soon discovers that Manny can't be found anywhere. He goes to talk to Manny's parents, but his house is completely empty. A dog with the same shaggy hair as Manny shows up and runs away. Larry, shaken that Manny or his folks did not say goodbye, knows that now the Geeks only have two guitar players instead of three.
Things go from bad to worse when Lily also disappears. He meets a dog that has the same multicolored eyes as Lily. When Larry tries to talk to her parents, they quickly drive away, but not before insisting that they never knew any Lily. Now down to one guitar player, the Geeks must make a difficult decision, as the Battle of the Bands approaches. Larry is now desperate to know what is in INSTA-TAN that made him grow hair. He is also curious, if the disappearances are related to it. Larry retrieves the bottle from the trash. Larry plans to take it to Dr. Murkin and have him analyze it to see if there is a cure. However, the stray dogs that constantly attack Larry chase after him causing him to drop the bottle and break it against a rock. Hopeless, the remaining children ultimately decide that regardless of the dwindling size of their band, the show must go on. Triumphantly, they declare that they will do it for Lily.
Finally, the night of the Battle of the Bands arrives. Since Howie and the Shouters went on first and were even called back for an encore, the bar was set pretty high for the Geeks. Larry is horrified to see that his entire face is covered in the hair and screams helplessly, knowing that everyone has seen his body hair. However, everyone thinks that it is all part of special effects. Larry flees the stage, runs home, and tells his parents what happened. His parent end up coming clean with the upcoming information:
- Dr. Murkin developed a way of turning stray dogs into children and then giving them to his employees. The twice-monthly injections were booster shots of the serum to keep the subject a human being. After a dozen years, the formula loses its effect. The INSTA-TAN had absolutely nothing to do with the hair growth. Everyone in the town works for Dr. Murkin but he decides that, after the serum's failure, he does not want to turn dogs into children anymore.
The book ends with Larry, now a dog, watching his parents bringing home a newborn baby girl named Jasper. The baby has Jasper's yellow eyes. This implies that Dr. Murkin has now found a way to turn cats into people.
- The Give Yourself Goosebumps book, The Deadly Experiments of Dr. Eeek, contains a quiz on this book. Interestingly, that book has one section where the reader becomes a dog, and indeed this quiz points him/her in that direction. It is not known, if R. L. Stine intended it to be ironic.
- This is the second time the protagonist is revealed to not actually be human. The last time was The Girl Who Cried Monster, and the third time was My Best Friend is Invisible.
- This book is one of a few times the protagonist does not succeed.
- Jared and Kristina are the only members of the band who don't turn into dogs.