Night Holds the Key
A story by Anachronism Girl, and semi-prequel to TAG's "April Is the Cruelest Month". She states the rating is "wavers somewhere between strong PG-13 and mild R for sexual content".
Daria, Jane, and Trent (holding down a day job) are living together following the car accident (from "April") that has crippled Jane. Even as Daria and Trent think it can't work out, the two of them stumble into a late-night encounter.
The title is a quote that Trent told Daria in "Jane's Addition". ("Neither of them had been entirely sure what he meant at the time.")
Winner of "Favorite Romance" in the 2nd Daria Fanworks Awards
"Night Holds the Key" at Outpost Daria Reborn