J1 was part of a group of surviving humans who had come to re-colonize one of their old planets.. J1 is military I believe, and Aldis is mentioned too.. they had dropped some sort of chemical do kill a previous group of humans, but it didn't work. Now they're mutated and become like their surroundings to hide. J2 is an alien (one of the mutated humans who were left on the planet), who makes a bargain with J1 - he will spare the group of students holed away in the cave if J1 will mate with him. J2 bites J1 starting the transformation of J1 into J2's "kind". They mate, the end.
I believe I read this on AO3 (searched there, unsuccessfully for a good hour) but can't remember. I also did a google/yahoo search.
Any help would be greatly appreciated!