Klaus is talking to Rebekah, telling her about Bonnie and how she is always on his mind . The rest is pretty much Klaus giving Bonnie an ultimatum 'choose me or your fire, be mine or you will burn'.
If you've seen the film Savages then this is the VD version of it with Bonnie as the main character. If you haven't seen it then you get the gist of it and it's a pretty amazing video :)
Kat talks about the prank @8:10 but doesn't say who pranks her...in the video of Joseph @ Eyecon he says it was him @11:00....PS Kat and Steve are ADORABLE! I will never ship Beremy but I wouln't mind KatQueen! LOL!
We never know when it begins. Klaus and Bonnie are enemies but they began to have conflicting feelings to each other.
Diclaimer: I own nothing, only my deep love to them.
Music: Adele, Skyfall