Max, who narrates this story in the first person, tells the reader that before he met Freak, he didn’t have a brain. This, he says, is “the unvanquished truth.”

Max first meets Freak, or Kevin, at daycare, the year that Max moves in with Grim and Gram, his mother’s parents. Freak uses crutches and calls himself “Robot Man.” One day, Freak leaves daycare and doesn’t return.

Max resumes the story in the summer before eighth grade. Grim, Max’s grandfather, worries that Max is beginning to look like Him, Max’s father. Max tells us he is L.D., learning disabled, and he doesn’t think he has much going on in his brain. One day, Max is in the backyard. He recognizes a woman moving items into a house down the block as Freak’s mom, and then sees Freak himself. Max is curious and he approaches the house. Freak asks the “earthling” (Max) to identify himself. Max runs back to his basement. He emerges later to find Freak trying to retrieve an object stuck high in a tree. Max helps him and they spend the afternoon together. Freak’s mom, who Max knows as “Fair Gwen of Air”, invites Max to dinner. Max learns that Gwen and his mom were friends years ago. 

Max and Freak go the Fourth of July fireworks show by the millpond together. Max hoists Freak onto his shoulders so that Freak can see. When the show ends, Tony D. (known as Blade) and his gang corner Max and Freak. With Freak steering Max by kicking him right or left, they run straight into the millpond, where Max becomes stuck in the mud. Blade and his followers throw rocks at them from the shore. The police arrive and rescue Max and Freak. One of the cops seems to recognize Max as the son of someone he calls “Killer Kane,” and asks if his dad is still in prison. Freak interrupts him and says, “We’re Freak the Mighty, that’s who we are.” 

Freak the Mighty spend the summer going on quests. On these quests, Freak uses words Max doesn’t understand and insists that Max look them up in the dictionary Freak always carries. Max pretends to understand.

One day, Max and Freak walk to the “Medical Research” facility where Freak says they have an Experimental Bionics Unit. Freak tells Max that he visits there every few months to be fitted for a bionic transplant. Max thinks such a procedure will be painful and dangerous, but Freak says that pain is a state of mind and life itself is dangerous.

The two boys embark on a nighttime quest to retrieve a purse that Blade and his gang tossed down a storm drain. The purse is empty except for an ID card with the address of a Loretta Lee on it. The next day, Max and Freak walk to an area of rundown apartment buildings called the Testaments. Loretta and a man named Iggy live at the address, and they both know Max’s father, Kenny Kane. Iggy says that Max is a “ringer” for his dad, Killer Kane. Loretta remembers that Freak is Gwen’s son. Iggy kicks the boys out and they run home.

The school year begins, and Max finds himself in the same class as Freak. Classmates yell slurs at him. Suddenly, Freak climbs onto Max’s shoulders and directs Max to walk around the room. “Freak the Mighty, Freak the Mighty!” he shouts. Soon the whole class is chanting. Freak the Mighty end up in the principal’s office. 

Friday the thirteenth in October comes and it’s a bad one. First, Max’s dad wants Max to write a letter to the parole board on his behalf. Max has a panic attack. Second, a medical emergency sends Freak to the hospital. His insides are growing but his outside isn’t.

As Christmas approaches, Grim tells Max that his dad is out on parole. Freak and Fair Gwen, Max, Grim and Gram spend Christmas Eve together. Freak gives Max a dictionary that he has written, filled with his favorite words. The best gift ever, Max thinks.

After the Christmas Eve festivities, Max’s father sneaks into the basement. He kidnaps Max, and they end up at Loretta and Iggy’s place. Kenny ties up Max in a vacant apartment and swears on a Bible that he did not murder Max’s mother. 

Iggy reports that Freak has sent the cops to look for them. Kenny escapes with Max to an abandoned building where he gags and ties Max up. When Kenny leaves, Loretta appears and works to free Max. No sooner is Max untied, than Kenny returns, and grabs Loretta to choke her. Max tries to stop him, yelling that he saw Kenny kill his mother. Kane lets go of Loretta and grabs Max’s neck. Seconds later, Freak appears with a squirt gun supposedly filled with sulfuric acid, which he sprays at Kenny’s eyes. Max and Freak escape, the cops arrest Kenny and save Loretta. Freak laughs because the squirt gun – filled in fact with soap, vinegar, and curry powder – worked. As Gwen arrives and carries Freak away, he yells, “Freak the Mighty strikes again!” 

The school year continues. Gwen warns Kevin that he has to be “extra careful.” He’s been having trouble breathing. Max awaits his father’s trial and is relieved when Killer Kane pleads guilty because he won’t have to testify. 

Freak, Fair Gwen, Grim, Gram, and Max celebrate Freak’s thirteenth birthday. Suddenly, Freak has a seizure and is rushed to the hospital. 

The doctors make an exception and allow Max to visit when Freak asks for him. Freak tells Max that he won’t be coming home looking the way he looks now, and that he’s having a special operation. He gives Max a gift of a blank book and tells Max to write in it, to tell the story of Freak the Mighty.

When Max arrives back at the hospital the next day, Freak has passed away. The doctor explains that his heart just got too big for his body. 

For a long time, Max does not come out of the basement. When he eventually does, he runs into Loretta Lee, who asks what he’s been doing. When he says, “Nothing,” Loretta tells him, “Nothing is a drag, kid. Think about it.” That night, Max takes out the blank book and starts writing “the unvanquished truth.” He finishes the story and feels good about having written a book. Max thinks maybe he’ll even read a book now.