Mom hands Calvin a letter he got in the mail. Calvin wonders who sent it. He never gets mail. There's no return address, only a skull with X's for eyes and its tongue hanging out. He wonders if it's the IRS.

Calvin shows Hobbes the letter he received. He notices it has a local postmark and wonders who would send a weird letter like that. Hobbes wonders if it's from a girl. Calvin drops the letter in horror, wondering if the post office doesn't screen anything.

Calvin isn't sure he wants to open the letter. It might be a mushy poem written with a pink pen in loopy letters. He sees the note is cut and pasted letters from a magazine. Hobbes reads the letter. It contains instructions for a code and says a coded message will follow.

Calvin thinks this is real secret agent stuff. He figures the message must be super top secret and important. He wonders what it will be and when he'll find out. Hobbes says it's a good thing Calvin is the patient type. Calvin is so excited, he has to use the bathroom.

Calvin sits at school with a smile on his face. He thinks it's exciting to get a secret message in the mail. He wonders if the message will arrive in the mail today. Miss Wormwood is trying to tell him something, but he sits with his smile. As Miss Wormwood continues talking to him without Calvin hearing, he wonders who sent it and why was it sent in code. Calvin sits in the principal's office with a smile on his face. He thinks he's growing up to be a spy, since he's so good at figuring out what's going on.

Calvin races inside the house wondering if he got a letter. Calvin opens the coded message. They decode it. It says "Calvin is a porridge brain". Hobbes says some people have secret admirers. Calvin has a secret detractor.

Calvin angrily crumples the coded message. Calvin wonders what kind of depraved maniac would go to so much trouble to insult him. He knows the twisted fiend has too much time on his hands. Mom hands Calvin another letter.

Calvin sees the skull on the envelope. It's from the secret insulter. This one reads "You look like a baboon and you smell like one, too! Ha. Ha". Calvin yells aloud wondering who could be sending these. Hobbes looks at Calvin and says a reckless exaggerator. He says Calvin doesn't look like a baboon. Calvin yells that Hobbes is a big help.

Calvin can't sleep. He wonders who's sending the insults and where it will stop. He's being driven crazy since there's no way to trace the lunatic. Calvin says this person has thought of everything. Mom looks at her cut up magazine and wonders who did it.

Calvin asks if he got another letter. Mom says when you write to yourself, you get a lot of mail. Calvin doesn't know what she's talking about. Mom says she knows he's been putting out the notes for the mailman every day. Calvin asks if she means the notes are coming from their house. Mom tells Calvin to ask before he cuts up her magazines. Hobbes sits with scissors and magazine in hand. From the other room, Calvin asks where the miserable bunch of stripey orange flea bait is.

Calvin chases Hobbes. He says Hobbes has been sending the insults. He chases Hobbes around a tree. He stops. He admits the skull drawings were pretty cool. Hobbes says you can tell a good spy by his ominous logo.

Calvin and Hobbes fight. Calvin says he'll never forgive Hobbes. Hobbes whispers something to Calvin. They shake hands. Calvin says it's funny. They're pals again. Susie gets a message cut from magazines saying she smells.