The novel “Fantastic Voyage” by Isaac Asimov was published for the first time in 1966.
Jan Benes is a scientist who has discovered a way to miniaturize matter for an indefinite time. Thank to agent Charles Grant, Benes manages to escape to the West but on his arrival he gets wounded in an assassination attempt by Soviet agents ready to die in order to prevent the USA from discovering how to improve miniaturization technology.
A quick medical examination shows that a blood clot in the brain is going to kill Jan Benes. The only hope to save him is to miniaturize a submarine and insert it into the scientist’s circulatory system so it can travel up to the clot and destroy it with a laser. The problem is that the miniaturization technology available to the USA allows to maintain that state for an hour and within the group sent to that mission there might be a spy.