For a three-act plot analysis, put on your screenwriter’s hat. Moviemakers know the formula well: at the end of Act One, the main character is drawn in completely to a conflict. During Act Two, she is farthest away from her goals. At the end of Act Three, the story is resolved.
Act I
Günter Grass makes it fairly easy for us to think of this book in three acts, since he divides the thing into three separate books. Book One gives us the scoop on how Oskar's mother was conceived by his grandmother, Anna Bronski, and grandfather, Joseph Koljaiczek. This back story is supposed to help explain Oskar's impulse toward destruction, since his grandfather was a serial arsonist. We learn about Oskar's miraculous birth as a newborn genius and his decision to stop growing when he's three years old. He's got something of a God-complex.
Act I gives us insight into Oskar's obsessive relationship with his drum and how it functions as a protection against the adult world. It's also in Act I that we get a glimpse of what is going on in Danzig in this prewar era. It shows us the conflict that Oskar deals with when his Jewish drum supplier is a casualty of Kristallnacht. Oskar's mother dies, and the Nazis are coming to power, but all Oskar can think about is where he'll get his new drums. Oskar is complicit in the death of his Uncle Jan Bronski, who might be his real father, when he forces Jan into a dangerous situation in order to find someone to repair his drum.
Act II
Oskar falls in love with Maria Truczinski, the beautiful young woman his father has hired to take his mother's place in the family store. While still having the body of a three-year old, he develops sexual feelings and assaults her while she sleeps. When Maria gets pregnant and marries Oskar's father Alfred. Oskar's heartbroken. He joins a traveling troupe of circus performers and becomes romantically involved with one of the other performers. After she's killed in an Allied attack, Oskar returns home to Danzig.
Shortly after he returns home, Russians invade Danzig and kill Oskar's father Alfred, who's a leader in the local Nazi party. Oskar may be responsible for his father's death. At Alfred's funeral, Oskar throws the drum into the grave and his body inexplicably starts to grow for the first time in over a decade. As his body starts to grow again, Oskar gets sick. He, Maria, and his "son" Kurt decide to leave Danzig for the city of Düsseldorf. Act II ends with Oskar's caregiver, Bruno, measuring Oskar's body and finding out that he's still growing.
Act III
After a long stay at the hospital, Oskar begins a new life in Düsseldorf. He gets himself a nice gig as a stonecutter's apprentice and is able to support Maria and Kurt financially. He also forms a pretty nutty obsession over a woman named Sister Dorothea who lives in his building. Oskar falls in love with her even though he's never seen her. One night he runs into her in the dark and tries to rape her. She leaves the boarding house and is later found murdered.
After a stint as a jazz musician at a nightclub, his old circus mentor convinces him to do a solo tour. His drum performances end up making him wealthy. Back in Düsseldorf, he's walking his dog, who runs into the woods and returns with a severed finger. It turns out to belong to the murdered Dorothea.
Someone witnesses him holding the finger and Oskar encourages him to turn him in to the police. He's tired of life and hopes to be sent to jail. After a half-hearted flight from the police, Oskar's arrested, convicted, and sent to a psychiatric hospital, where all his needs will be taken care of. Act III ends with Oskar in the asylum, reflecting on how death has always followed him everywhere he's gone.