Now, Others Believe Him ... But Will It Be Too Late?
Why it is hard sci-fi
A 1967 series in which an architect realises that aliens have begun to infiltrate Earth as look-alike humans. The show commits to a single procedural. The aliens have small physical tells, including a permanently bent little finger and a tendency to disintegrate on death rather than leave a body. The protagonist runs each weeks investigation as a covert intelligence operation against an unbelieving public.
Science inside it
Look-alike infiltration as the central biology, a working set of physical tells that hold up across episodes, and an early portrayal of the procedural shape of a one-man counter-intelligence operation against a hidden enemy. The shows lo-fi rule of disintegration on death is its way of preventing forensic confirmation, which the show acknowledges as the antagonists deliberate strategy.