The story revolves around the life of Oskar Matzerath, as narrated by himself when confined in a mental hospital during the years 1952–1954. Born in 1924 in the Free City of Danzig (now Gdańsk, Poland), with an adult's capacity for thought and perception, he decides never to grow up when he hears his father declare that he would become a grocer. Gifted with a piercing shriek that can shatter glass or be used as a weapon, Oskar declares himself to be one of those "clairaudient infants", whose "spiritual development is complete at birth and only needs to affirm itself". He retains the stature of a child while living through the beginning of World War II, several love affairs, and the world of postwar Europe. Through all this, a toy tin drum, the first of which he received as a present on his third birthday, followed by many replacement drums each time he wears one out from over-vigorous drumming, remains his treasured possession; he is willing to commit violence to retain it.
Oskar considers himself to have two "presumptive fathers"—his mother's husband Alfred Matzerath, a member of the Nazi Party, and her cousin and lover Jan Bronski, a Danzig Pole who is executed for defending the Polish Post Office in Danzig during the German invasion of Poland. Oskar's mother having died, Alfred marries Maria, a woman who is secretly Oskar's first mistress. After marrying Alfred, Maria gives birth to Kurt, whom Oskar thereafter refers to as his son. But Oskar is disappointed to find that the baby persists in growing up, and will not join him in ceasing to grow at the age of three.
During the war, Oskar joins a troupe of performing dwarfs who entertain the German troops at the front line. But when his second love, the diminutive Roswitha, is killed by Allied troops in the invasion of Normandy, Oskar returns to his family in Danzig where he becomes the leader of a criminal youth gang (akin to the Edelweiss Pirates). The Red Army soon captures Danzig, and Alfred is shot by invading troops after he goes into seizures while swallowing his party pin to avoid being revealed as a Nazi. Oskar bears some culpability for both of his presumptive fathers' deaths since he leads Jan Bronski to the Polish Post Office in an effort to get his drum repaired and he returns Alfred Matzerath's Nazi party pin while he is being interrogated by Soviet soldiers.
After the war Oskar, his widowed stepmother, and their son have to leave the now Polish city of Danzig and move to Düsseldorf, where he models in the nude and works engraving tombstones. Mounting tensions compel Oskar to live apart from Maria and Kurt; he decides on a flat owned by the Zeidlers. Upon moving in, he falls in love with Sister Dorothea, a neighbour, but he later fails to seduce her. During an encounter with fellow musician Klepp, Klepp asks Oskar how he has an authority over the judgement of music. Oskar, willing to prove himself once and for all, picks up his drum and sticks despite his vow to never play again after Alfred's death, and plays a measure on his drum. The ensuing events lead Klepp, Oskar, and Scholle, a guitarist, to form the Rhine River Three jazz band. They are discovered by Mr. Schmuh, who invites them to play at the Onion Cellar club. After a virtuoso performance, a record company talent seeker discovers Oskar the jazz drummer and offers a contract. Oskar soon achieves fame and riches. One day while walking through a field he finds a severed finger: the ring finger of Sister Dorothea, who has been murdered. He then meets and befriends Vittlar. Oskar allows himself to be falsely convicted of the murder and is confined to an insane asylum, where he writes his memoirs.
