A romance, sexual or otherwise, between two different species. Sometimes they are humanoid aliens, and a Green-Skinned Space Babe is involved, or they can be a Friendly Neighborhood Vampire and a human, two Funny Animals, or a Beast and Beauty couple. The type of species can diverge greatly, so long as they are different. For some this is the way to go about First Contact. Alien invaders seemingly love to tempt humans with this as part of their Masquerade, although you'd think they'd learn better.