A biochemist, Rose Smollett, accepts a non-human visitor to Earth, named Harg Tholan, into her home as a guest. Her policeman husband, Drake, is suspicious and hostile, and her investigation into why leads to the discovery that humans— including her husband—are hosts to a parasitic life-form which is deadly to all the non-human races of the Galaxy.

As with many other stories in Nightfall and Other Stories, this one not only involves a non-human race, but in a fashion where humanity is inherently different from all other intelligences. Unlike the sort of Earthman/alien conflict story John Campbell was so fond of, however, humanity isn’t superior here but decidedly dangerous.

I do like this story a great deal. The three main characters are all well-drawn, and the separate mysteries involving them rather explicitly laid out until they reach an unexpectedly intertwined solution.

There are, by the way, two textual variants to this story. The version in Nightfall and Other Stories is naturally the better known; but the obscure anthology Other Worlds of Isaac Asimov reprints the story as it actually appeared in Horace Gold’s Galaxy. (Gold was also running a story with a similar theme by Theodore Sturgeon and asked Asimov, as the author of the weaker story, to make alterations.) The parasitic intelligences of the story are turned into mindless pseudo-genes, a new class of infection, and the story includes two additional paragraphs at the stories end. You don’t notice they‘re missing unless you know they‘re missing, but since I imagine few people have access to Other Worlds of Isaac Asimov, I’ll reproduce them here immediately after the ending we all know and love:

"She had finally learned why Drake had married her.

"Not a conjugal relationship—

"Conjugation".

Strangely, this is not Asimov’s original ending, which ran:

"She had finally learned why Drake had married her.

"She was laughing--- quite loudly---"

Frankly, Gold’s ending is the weakest of the three, but I'm not sure that the original ending is better than the one Asimov usually used.