A trip back in time from the present to...
Why it is hard sci-fi
A man uses an unattended time machine in a neighbours lab to travel one hour into the past. The film commits to the rule. There is a single timeline. Every loop is consistent with itself. The protagonist is forced to perform every action he has already witnessed, with no escape clause. Vigalondo wrote it as a puzzle and the puzzle solves on the second viewing.
Science inside it
A single-timeline closed loop with no branching, the operational mechanics of a time machine that requires the user to spend roughly an hour in the past to return to the present, and the inevitability of becoming the original cause of every event you intended to undo.
Spoiler alert
The protagonist becomes his own pursuer, his own attacker, and the cause of his wife s death. The film closes on a third return where he watches a younger version of himself running into the lab, and accepts that the loop will continue without him.