Person A and Person B have not met yet, but they happen to be sitting next to each other at a bar when some jerk starts making unwanted advances to Person A.

Person A then turns around and tries to pretend Person B is their girlfriend/boyfriend/other. Person B plays along, and when the guy finally leaves, they strike up a conversation.

 A few minutes later, one of them discovers that something had been slipped into Person A's drink and the two decide to hunt down that asshole.