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 "auditory clairvoyant babies", 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 tin drum that he received as a present on his third birthday remains his treasured possession, and he is willing to kill to retain it.
Oskar considers himself to have two "presumptive fathers" - his mother's husband Alfred, a member of the Nazi Party, and her secret lover and cousin Jan, 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 Oskar's possible son, Kurt. 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. The Russian 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 moves with his widowed stepmother and their son to Düsseldorf, where he models in the nude with Ulla and works engraving tombstones. Oskar decides to live apart from Maria and her son Kurt after mounting tensions. He decides on a flat owned by the Zeidlers. Upon moving in, he falls in love with the Sister Dorothea, a neighbor, but he later fails to seduce her. During an encounter with Klepp, Klepp asks Oskar how he has an authority over the judgement of music. Oskar, willing to prove himself once and for all to Klepp, a fellow musician, 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 and Oskar and Scholle (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.