I'm a gigantic nerd, so when the opportunity popped up to read Lilly Cain's Undercover Alliance on NetGalley, I jumped at it. This Carina Press erotic novella details a human-alien relationship between Sarina and the man she's assigned to protect, John Bennings.When the story begins, Sarina is damaged; her external nerve system that defines her race has been severed, and she's unable to reach a sexual climax, which her people determine is necessary to their mental health. As a result, this female warrior is about to be put out to proverbial pasture after she completes one last assignment: protecting a lawyer who's working on the alliance contract between humans and her people.Of course, things aren't as they seem, and John is actually undercover, assigned to make sure the contract is signed, and everyone involved is protected. And if he's attracted to his bodyguard? Well, he probably shouldn't act on it. Too bad Sarina's people are matter-of-fact about sex, and she can see the sexual fantasies about her through her mind-reading ability.Undercover Alliance is a solid three-and-a-half stars. The world-building is excellent, and the relationship between the two characters is believable, even if the novella length makes it feel a bit rushed. I'd have liked to have seen this story in a full novel length, with more tension between the two characters for a bit of time, and perhaps more explanation of John's position; the opening scene, for instance, never did seem to make complete sense to me.The only other quibble I have with the book is that the alien language falls into the trope area, with a bit of overuse and the ubiquitous apostrophes that often trip up sci-fi fans. However, there is a good balance between the plot and the sex scenes (which are actual plot movers in this one due to Sarina's presumed disability), and the combination of good sci-fi world-building with the erotic scenes led me to round up instead of down.I'm looking forward to reading the previous two books Cain has set in this world as a result.