Maniac Magee is about Jeffrey Lionel Magee's search for "home" and the racial prejudice in the town of Two Mills. Spinelli has written the novel in third person ("He said," as opposed to "I said," which is called first person), using an objective viewpoint (in which the thoughts and feelings of the characters are not revealed).
Jeffrey lived in Bridgeport with his parents until he was three years old. During that year he became an orphan. His parents were killed in a trolley accident over the Schuylkill River. During the next eight years, Jeffrey lives with his Aunt Dot and Uncle Dan in Hollidaysburg, Pennsylvania. The household is extremely dysfunctional because his aunt and uncle never speak to each other. They dislike each other but, being strict Catholics, refuse to divorce. After being bounced back and forth between his aunt and uncle, Jeffrey reaches his breaking point. At the spring musical at Jeffrey's school (Jeffrey is in the chorus), which his aunt and uncle attend, he screams at the top of his lungs, "Talk! Talk! Talk!" and then runs away.
Jeffrey begins his search for a new home. He runs for a year, and with his sneakers falling apart, he ends up in Two Mills, a town linked to Bridgeport (the town he grew up in) by the Schuylkill Bridge. The first day he is in Two Mills, he makes four appearances. First, he speaks to a black girl, who appears to be close to his age, named Amanda Beale. She has a suitcase that he discovers is full of books. He borrows a book from her and promises to return it.
Jeffrey's second appearance is at the high school football field. He catches a pass meant for Hands Down, a receiver on the football team. After running all the way to the soccer field, he performs one of his amazing feats — he punts the football back to the football team, a punt that travels farther than any of the football players has ever thrown it, all the while holding onto his borrowed book. Next, he rescues a 10-year-old boy from Finsterwald's backyard, a place that causes terror in all the kids in West End, and then responds to Mrs. Pickwell's whistle when she calls all ten of her children home for dinner. He sits down to a spaghetti dinner with the large family without anyone asking any questions. The next time Jeffrey is noticed is at a Little League game. He again performs incredible feats by hitting John McNab's fastball and bunting a "frogball" for several home runs. Whenever anyone spots Jeffrey he is running — fast. Stories begin to circulate about Jeffrey's amazing feats earning him the nickname "Maniac."
Jeffrey learns that Two Mills is segregated — the East End is where the black people live and the West End is where the white people live, and the dividing line is Hector Street. After being chased by John McNab and his group of friends, the Cobras, Maniac ends up in the East End and is confronted for the first time by Mars Bar. Maniac is oblivious to the racial differences that exist between them. Rescued by Amanda Beale, Maniac goes to the Beale house. After revealing to Mr. Beale that his home is the deer shed at the zoo, the Beales invite Maniac to stay with them. He finally has a home with an address.
Maniac is happy with his life. He fits in with the Beales just like he belongs there; however, he is unaware of the prejudice that exists between the blacks in the East End and the whites in the West End until an old black man calls him "Whitey" and tells him to "go home." The situation escalates when graffiti is written on the side of the Beale house ("Fishbelly go home.") and when Amanda's coveted Volume A of the encyclopedia is stolen, cut up, and used as confetti when Maniac untangles Cobbles Knot. When he realizes that his presence in the Beale house is hurting the Beales, Maniac walks down the center of the street, between the blacks and the whites, out of town.
Maniac is found, hungry, scraped up, and dirty in the buffalo pen at the zoo by an old man named Grayson. Grayson takes him to the baseball equipment room in the band shell. He feeds Maniac, buys him some clothes, and lets him stay in the baseball room. Maniac and Grayson become close friends. As they get to know each other, Maniac finds out that Grayson played baseball in the minor leagues and doesn't know how to read. Maniac teaches Grayson how to read and Grayson teaches Maniac about baseball. Maniac also enlightens Grayson about black people, telling him that they are just like white people, a surprise to Grayson. Maniac and Grayson are happy together and once again, Maniac has a home with an address he makes up: 101 Band Shell Boulevard. They happily spend the Thanksgiving and Christmas holidays together, and five days after Christmas, Grayson dies in his sleep. Maniac is alone again. To deal with his grief, Maniac runs wherever his legs will take him.
Depressed and lonely, Maniac ends up at Valley Forge where he decides to die. He refuses to allow anyone else to orphan him. The second night Maniac is there in a small cabin, two small boys show up, Piper and Russell McNab, brothers of John McNab. The young boys are running away from home. Maniac takes the boys back to the McNab house and ends up staying there. The house is dirty and messy and has cockroaches crawling everywhere. Maniac performs "heroic feats," to persuade Piper and Russell to go to school.
Piper and Russell dare Maniac to go to the East End (they don't know he used to live there). Maniac uses the dare to get the boys to go to school. While walking through the East End, Maniac encounters Mars Bar, who insists on racing him. A crowd gathers and they race. Maniac wins, beating Mars Bar and running the last few feet of the race backwards. Maniac regrets humiliating Mars Bar by running backwards; however, his actions cause Mars Bar to hate him (an individual) rather than white people in general.
At the McNab house, the Cobras (John McNab's group of friends) are building what they call a "pillbox" in the middle of the living room. The "pillbox" is a fortress to protect themselves against blacks. John envisions blacks from the East End attacking the West End all at once, probably in the summer. The idea of the fortress and the prejudicial attitude that prevails within the McNab household causes Maniac to feel dirty.
Maniac leaves the McNabs, choosing to sleep in the park. He could-n't stand what the McNabs were doing or the way they were thinking. Russell and Piper find him in the library and insist Maniac come to Piper's birthday party. Maniac agrees — as long as he can bring a friend. His idea is to bring Mars Bar and his intention is to teach the McNabs that blacks are human beings just like themselves, and to teach Mars Bar the same thing about whites.
Mars Bar accepts Maniac's challenge and courageously goes to the West End with Maniac. They go to the Pickwells' house to eat, and Mars Bar is readily accepted by the entire family. Then they go to the McNabs' house, where John and the Cobras try to intimidate Mars Bar. Maniac has to drag Mars Bar out of the house to prevent a fight from erupting. Mars Bar asks Maniac, "Wha'd you think?" When they part, Mars Bar is angry with Maniac.
Eventually, Maniac and Mars Bar begin to run together in the mornings and learn to respect each other. Mars Bar rescues Russell McNab from the trolley trestle (the site of Maniac's parents' death) and takes him home — to the West End. In his way, Maniac succeeds in breaking down barriers between blacks and whites. Mars Bar and Amanda find Maniac in the buffalo pen at the zoo and insist that he return to the Beales'. Maniac finally feels as though he is going home.