The Rooftop Garden is a novel about Nabila, a researcher who studies seaweed in warming oceans, and her childhood friend Matthew. Now both in their twenties, Matthew has disappeared from his Toronto home, and Nabila travels to Berlin to find him and try to bring him back.
The story is interspersed with scenes from their childhood, when Nabila, obsessed with how the climate crisis will cause oceans to rise, created an elaborate imaginary world where much of the land has flooded. She and Matthew would play their game on her rooftop garden, the only oasis in an abandoned city being claimed by water.
Their childhood experiences reveal how their lives are on different trajectories, even at an early stage: Nabila comes from an educated, middle-class family, while Matthew had been abandoned by his father and was often left to deal with things on his own.
As an adult, Matthew's dissatisfaction with life leads him to join a group of young men who are angry at society. He eventually finds himself on a violent suicide mission, but Nabila isn't aware of the extent of his radicalization until they finally meet on a street in Berlin.