I've been searching for hours and can't find one of my favourite frostiron fics, so was hoping someone could help me out.
Basic premise, set post Avengers, Loki has been sent back to earth as punishment for his crimes. Whilst out inspecting craters (no really) he, Tony, Thor and Steve get abducted by aliens. Tony and Loki end up in a cell together, and fake Loki raping Tony in order for Loki to get close enough to Tony for long enough to keep him alive (the aliens have stolen Tony's arc reactor).
I pretty sure it was the first in a series.