There's a door, somewhere deep in a cavern inhabited by vile and evil things, that can only be opened by a truly good act. It's a door to another plane: one so sacred and holy that only the pure of heart can walk in it. I just wondered what kind of act could be so pure that it would open the door, that's all.
I had this image of a paladin who has been banished from his order because he fell in love (and slept with his girlfriend: knights of the order are supposed to be celibate, though they're allowed to marry). He's so depressed by the banishment that he hasn't tried to use his abilities since: he thinks they're gone. Yet he and his friends get to this door, deep underground, sweaty and scared and pursued by evil things they can't possibly beat. And when he takes his lover in his arms, the doors open...