At the end of Frankenstein; or, the Modern Prometheus, the creature flees to the Arctic and perishes. In X-Men #40, it is supposed that this has really happened, and an expedition sent by New York's City Museum has found him frozen in ice, and transported him back. Freezing, as we know, is not fatal in Marvel.
Xavier has found out about this and wants to go investigate. He says he has always been sure that Shelley's novel was based on an actual occurrence (?), and further suspects that the 8-foot tall humanoid described therein is an android. This provides the workaround for the CCA. He also reckons that the android was made by a mutant in the 19th century (by someone like Forge, I suppose), which superficially provides the reason why this is even an X-Men story in the first place.
But "X-Men Meet Frankenstein" shouldn't need that justification. There is a story to be told here, comparing the tragedy of Frankenstein's monster with the that of the X-Men. Noble souls driven to the edge by society's inability to accept their physical differences. This book sadly ignores all that, and treats it not as Shelley's wretch, but as a cross between Karloff's monster and a Hulk angry at the puny humans. It perhaps has suffered from the truncated length (15 pages) in the new format with the "Origins of the X-Men" backup strip...
The X-Men and the monster fight. Rather appropriately, Iceman is able to freeze the monster again. We get a small wrap-up, with Professor Xavier explaining what he saw when he eventually penetrated the android's (for it was indeed an android) brain. It was not built by a mutant at all, but was an ambassador built by an alien race, strongly resembling the 1930s movie version of the monster. It went wrong, attacking humans, then, as now. The X-Men's costumes resembled its brightly-coloured creators who tried to shut it down. Shelley somehow found out about this. None of this makes any sense at all, but is enough to place it into science fiction rather than horror. Still, they're skirting around the edges of acceptability. The prohibition on horror comics won't last long now.