Asimov's original novelization of the 1960s film was good, but someone else's idea. This is his revisiting of the same concepts 25 years later in a novel that he describes as his own, almost in entirety. Over all not a bad read.
In a nutshell, the Russians have the technology to miniaturize anything, but at a huge and impractical cost in power. They kidnap an American "neurophysicist", whose ideas of interpreting the electrochemistry of thought have been widely ridiculed and rejected in the states. He is to be a part of a desperate plan to miniaturize a small submarine with a crew of five to be injected into the brain of a comatose patient, specifically the very scientist who possessed the secret of making the miniaturization process economically practical. If the discredited American's theories are correct he might help the Russians recover that secret from the unconscious but still functioning brain of the patient. It is also a chance for him to prove his own theories once and for all.