Until Mary's grandmother reveals that she does, in fact, have angelic ancestry and activates her potential as "an engagement present."
Parodied: Mary and Tom love one another, but their mutual misunderstanding always ends up causing wacky hijinks and physical violence.
Zig Zagged: Mary falls from the sky and Tom is convinced she's an angel sent to him to help him deal with how little of a life he has. He takes her into his apartment and nurses her back to health; by the time she's better, it's become clear she is in fact no angel. However, by this time, they've grown on each other and so much wacky stuff has happened that they decide to stick with the facade - and one wacky adventure does turn Mary into an angel. But the thing is that angels lose their powers if they fall in love...which turns them into cupids and Mary's new duties cast a fresh light on their relationship.
Averted: Angels either don't exist or stay in heaven where they belong. Tom gets over his fantasy of the perfect girl and meets somebody nice.
Enforced: When all you have is a geeky 12-24 year old male audience....
Lampshaded: "Mary's wonderful girl but there's something odd about her....Wait, is that a halo!?"