The thing that won't die, in the nightmare that won't end.
Why it is hard sci-fi
A 1984 film with a single hard idea executed with discipline. A combat cyborg is sent back through a one-way time displacement device to assassinate a target before her child is born. The film commits to the loop. The displacement device requires a living biological envelope, no machinery survives the trip, and the films signature scene is the cyborg arriving naked. Every constraint of the trip is shown on screen.
Science inside it
A single time-displacement event with strict cargo rules. Cybernetic prosthetics, neural-net controllers, and the operational logistics of a humanoid combat unit working from incomplete intelligence in a foreign era. The film also carries a working causal-loop argument. Reese is the targets future husband, and the film owns the implication.