Fantastic Voyage by Isaac Asimov, based a screenplay by Harry Kleiner, adaptation by David Duncan, based on a story by Otto Klement and Jay Lewis Bixby (1966)
A famous scientist with information vital to the survival of the Free World is injured in a failed assassination attempt. In order to save his life and the knowledge he holds, a medical team is reduced to the size of a microbe and injected into him in to carry out an impossible brain operation.
This novelisation of the 1966 screenplay is famous because the production delays went on for so long that Asimov's version came out months before the film did. Because of this, the myth was born that the film was based on the book despite the very clear credits on the front cover. Asimov didn't really care much for the script. He felt that it was scientifically inaccurate and full of plot holes and insisted that he be allowed to write it as he saw fit. The studio wasn't too bothered about the novelisation, so they let him have his way. The result is Asimov's only science fiction novel of the 1960s; a time when he'd turned his hand almost entirely to writing popular science.
When a film is turned into a novel, it's usually a harkening back to the days before recordings and on-demand technology was available; a time when a book was the only alternative to memory and the Late Movie. At best, a novelisation adds to the film by expanding it, giving us more insights into the character's minds or even affording the opportunity for a last rewrite before history declares its verdict.
Fantastic Voyage is Asimov's crack at the latter. In some ways, this is Asimov's weakest novel because it isn't his own story and he'd a bit disgruntled about it. In others, it's his best because he's working from images given to him by the studio, so his descriptions are, for once, up to par. He tries to expand on the characters, but the more he opens them up, the smaller they become. The secret agent Grant goes from professional and heroic to a mess of fed up combined with adolescent boorishness. Cora goes from an attractive woman with a crush on an older man to one of Asimov's weakling Feminists. Duval is still a deeply religious man, but since Asimov wasn't, he has no idea how to handle this. Worse, Asimov was entirely adverse to reusing script dialogue, so the conversations are now rambling and pedantic.
As to filling the plot holes, Asimov is constantly doing so to either holes that are not germane to the plot or that weren't plot holes in the first place as Asimov keeps second guessing the script with far too many ifs and buts tossed like a salad. Even dealing with the famous one of the destroyed submarine being left behind is done in a way that only makes matters worse. He just couldn't understand that if you're in a corner like that and you can't get out by a total rewrite, then you just have to brass it out and rely on refrigerator logic.
The reasons behind the shortfalls are threefold. First, Asimov was a bit of a pedant himself and wanted to conduct the tour of the human body himself. Second, Asimov loved whodunnits that end with a long-winded, genteel explanation of events so that he bent the script in the service of one. And third, the Cold War plot with the Americans as the goodies and the Soviets as the baddies didn't sit well with Asimov, who was quite left-wing and I suspect flirted with Communism in his youth. According to FBI records, the Communists certainly thought him a likely recruit. So, the "who is the assassin in the sub?" subplot, which was as straightforward as you can get, is dumped in favour of one with needless red herrings and the villain turning out to be not a cold-blooded KGB agent bent on murder at any costs, but a conscientious scientist who is worried that the US military would become too powerful, subvert the miniaturisation project away from pure science and probably start World War III, so he snaps and acts as a lone agent and all that assassination stuff is mere window dressing by coincidence.