Abstract Rules of Engagement

Ownership; 9/10ths of the law. Right? What happens when a pudgy looking feline shows up on the doorstep of a professor and his flatmate? What happens when the owner seeks the cat only to find him in the window of a stranger's house? No collar. No evidence. Will the young catlover ever get her feline companion back from the swift tongued animal enthusiast and his bookish friend? AU!