Nothing is real, everything is possible.
Why it is hard sci-fi
A 118-year-old man is the last mortal human in a society that has cured ageing, and his recollection branches across multiple incompatible lives. The film uses the multiverse as a literal narrative tool. Each life is a self-consistent thread the protagonist could have lived, and the films grammar lets the threads collide. The science backbone is loose but the rule about each branch being internally consistent is followed.
Science inside it
Many-worlds branching as an explicit storytelling device, life extension, and the films interest in what a life looks like when reproductive cell senescence has been switched off. The opening lecture about pigeon superstition is a real Skinner experiment and serves as the films thesis on causal reasoning.