D.O.A. (1949)
A small-town accountant walks into a police station to report a murder — his own. Slipped a "luminous toxin" with no antidote, he has a day or two to trace the poison back through nightclubs, blackmail and a shipment of stolen iridium before it kills him. Told entirely in flashback from a dead man walking.
The purest ticking-clock premise in noir: the detective and the corpse are the same man, and the clock never stops.
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