Fantastic Voyage II: Destination Brain is not a sequel to the 1960s Asimov Fantastic Voyage novelization of the 1960s movie. Asimov was not satisfied with the science and some other aspects of that story. So, in the late 1980s, he wrote his version of the basic idea: miniaturize humans in a miniaturized ship and send them on a voyage through a small part of the body, which because of their size would seem like an Odyssey of a voyage. Thus, this new version of the story retains nothing else from the original story, no characters, settings, or fundamental plot details. Similar scenes, such as the terrifying trip by one person outside the ship, are entirely reimagined.
The plot plays out as a kind of political thriller. Published in 1987, Fantastic Voyage II portrays a world 100 years in the future that is little changed from 1987. The US and the Soviet Union are not exactly at each others' throats any more, but still maintain a pointless and dangerous game of political and social oneupmanship that looks very much like it had been from the 1950s to the 1980s. Technologically as well as politically, the world of the 2080s is little different from the world of the 1980s. The Soviets kidnap an American scientist on the skids, Albert Morrison. He works in neurophysiology and has a largely discounted theory that there are nodes of thought within the brain. He also believes that he has discovered a pathway to limited telepathy, though he keeps that idea mostly to himself. The Soviets kidnap him, with American government approval, to take him to their miniaturization project. The Soviets want him because they are planning a journey of miniaturized humans into the brain of the comatose and dying master scientist Shapirov, and want Morrison to use his techniques to recover any scientific thoughts that might still be lurking in Shapirov's degenerating brain.
One purpose for all this is to make Fantastic Voyage close to a hard science brand of science fiction. The theoretical aspects of miniaturization are gone over in broad terms, with a semi-plausible explanation for its accomplishment provided, shrinking the Planck constant. There are some speculations about the relationship between the Planck constant and the speed of light. Much of the dialogue in the story is devoted to the general physics related to miniaturization. Asimov introduces a second bit of scientific speculation, the idea that brain waves contain information that can be in some way broadcast like radio waves, thus explaining telepathy. This point seems on shakier grounds scientifically, and so less is made of it in the scientific arguments that pass for dialogue, though more is made of it in the plot.
Some of the problems with the story, as far as I am concerned, are in the dialogue, plot, science, and characterization. The dialogue gets rather tedious as Morrison constantly goes back and forth with the Soviets about American science versus Soviet science, and about the politics of nationalized science programs and state secrets. The plot does not bear much scrutiny. Would the Soviets, or any government for that matter, spend the obviously huge amounts of money on an ill-conceived and mostly unscientific attempt to retrieve stray thoughts from a dying scientist no matter how much of a genius he might be, using unproved and discredited methods? The science in theoretical terms is fine, consonant with the science fiction theme of taking an idea on the edge of possibility and seeing what one can do with it. The "experiment" itself, though, is almost entirely unscientific. There are no controls, no real recording devices or measuring devices, and the claim that telepathy might be happening rests entirely upon a person's say-so. The characters are rather one-dimensional and get tiresome in that they offer no surprises for the reader. Morrison is a cowardly dweeb, whining and whimpering when he's not fainting or complaining. Boranova is overbearing and manipulative, Donev is selfish and arrogant, Kaliinin is devious and romantic, and Dezhnev can't go for five words without invoking some "saying" by his father. 300 pages of this just gets on my nerves.
One redeeming aspect of the novel is that Asimov has reserved a twist in the plot. The real purpose of the story is not to justify the ideas of miniaturization and telepathy. The real purpose is to solve the Cold War, which in 1987 was still a very real thing, though only five years away from vanishing. The real intention had been in the dialogue all along. All the back and forth about Soviet and American science and government intentions and so on, which seemed just there because the kidnapping of an American scientist made it seem necessary to be there, was the main point. Spoiler here, so if you don't want to know, skip here. The twist is that Morrison discovers that he was right, but not in the way he thought he was. He also realizes that the competing fields of science, miniaturization for the Soviets and telepathy for the Americans, will each require the other. Thus, practicality of perfecting the sciences will render moot the competitiveness of keeping scientific advancement a state secret. The two countries must cooperate; therefore, Cold War over. This twist makes relevant all that political bickering that for the duration of the novel felt to me overdone and irrelevant. It reveals, though, that Asimov, at least in this novel, was ludicrously politically naïve.
In the end, then, a mixed review for Fantastic Voyage II: Destination Brain. It makes for a good artifact of the late Cold War. As far as reading enjoyment goes, it's up and down.