Your heroine is at a Halloween Party. She’s in costume and no one recognizes her, not even her boyfriend who she sees doing something he shouldn’t be doing. What is he doing? What is her reaction? And who makes her feel better?
For example, maybe she sees her boyfriend hitting on her best friend. She runs out of the party and runs smack into a handsome stranger who offers to buy her a drink and help her forget the lout she just left. Or maybe she sees her boyfriend making an illicit and illegal deal with someone and an undercover cop comes to her aid.