Your mind is the scene of the crime
Why it is hard sci-fi
A heist film with a working dream-sharing technology called the PASIV device. Nolan commits to a stack of rules. Time dilates by a factor of about twenty per layer of dream. A kick at one layer wakes the dreamer one layer up. Limbo is unstructured space at the bottom of the stack. The films grammar is rigorous and the climax cross-cuts four layers in real time without violating the rule.
Science inside it
Multi-layer simulation as a working stack, time dilation across layers, the totem as an authentication device for which level you are on. The film also leans on real ideas about dream incubation and lucid dreaming, even where it pushes them past their evidence base.
Spoiler alert
The final spinning-top shot is left ambiguous on purpose. Nolan has said the question is not whether Cobb is in a dream but whether he still cares. The audience never sees the top fall, and the film treats that as a sufficient answer.