I am working my way through TOS for the first time, and last night I watched this episode, so the Tarsus IV references finally make sense to me. And wow, what an amazingly powerful fic this is. I think my favorite part was the one with Kevin -- I felt like I was barely breathing as I read it. And those last lines...

In the morning Kevin is gone before Jim wakes up, and when the transfer request crosses his desk a few days later he approves it without thinking of why.

Some things you just can't live with other people. Some ways you just can't bear to be known.

Amazing.