Straight on till Morning features the Avengers characters as Star Trek officers. The characters are perfectly themselves, yet the Star Trek world is also perfectly believable. It's just all so well done and such a fun story!
There were two small downsides for me. One was there was a (brief) sex pollen scene. Sex pollen never really works for me, and in this case it wasn't really even necessary for the story. I skimmed the sex itself during that scene. The other was also sex-related: I skimmed/skipped the big sex scene as well. I'm not sure if sex in fics doesn't work for me anymore or if it was some other reason. Whatever the cause, the story is just as readable without those two scenes.
But those were two minor things. The writing was very, very good, and the story hooked me hard. Recommended!