crossing the line

"It seems to me that the best relationships, the ones that last, are frequently the ones that are rooted in friendship. You know, one day you look at the person and you see something more than you did the night before. Like a switch has been flicked somewhere. And the person who was just a friend, is suddenly the only person you can imagine yourself with."

In short, Steve and Natasha were best friends. Until that one night, one minuscule decision, a momentary lapse in judgement, changed it all.