Max, the narrator, relates the events of his life during 8th grade. Because he's exceptionally large for his age and because his dad -- called Killer Kane -- is in prison, Max is not accepted by his peers in school. He makes friends with a similarly displaced kid named Kevin, whom Max refers to as "Freak." They bond over their mutual loneliness, Freak being rejected for his small stature and frequent absences. In coded language, Freak explains that he's ill and hoping for an operation which will give him robotic legs. Together they form a duo where Freak rides around on Max's shoulders, calling themselves collectively "Freak the Mighty."
One day Max and Freak return a woman's stolen purse. Unfortunately, the purse's own, Loretta Lee, is a con-artist who causes trouble for the boys. Her boyfriend, Iggy gets involved. He recognizes Max because he's an old friend of Killer Kane. At this point Freak learns the truth about Max's dad for the first time. He's cool about it. When Max's dad gets parole, he returns home on Christmas Eve and kidnaps his son. Thanks to Freak's help, the police rescue Max, but he's traumatized by the affair. He reveals that his dad is in prison for the murder of Max's mom.
For his birthday, Freak receives a computer. He's going to take school classes from home from now on. Max doesn't understand why this is necessary, but no one at school will answer his questions. Then Freak has a seizure and spends some time in the hospital. When he visits his friend, Max receives a journal and instructions to record the friends' adventures from the past year. Max returns to the hospital the next day only to learn that Freak has died. He had lied about the potential operation, trying to give his friend hope.
The next year is especially difficult for Max. He's worse off now that he knows what having a friend is like and knowing what it's like to lose that friend. Writing in the journal which Freak gave him before his death, Max works through his depression and suffering cathartically. By the end of it, he's written Freak the Mighty.
