Bert has just taken a job as an assistant to divorce lawyer Stephanie Whitman, an overbearing, humorless micro-manager who demands that Bert work on Saturday. But since Bert already has plans, and Stephanie won't even be in the office on Saturday, Nan agrees to take over desk duty for her sister.

The last thing Nan expects is a too-dose encounter with a surprisingly-well-dressed burglar fleeing Stephanie's office. Nan calmly calls the police, continuing her impersonation as she describes the intruder. But on Monday when the cops have a suspect, it's Bert they take to the station. Unable to identify the culprit, Bert watches helplessly as the suspect is released. And when she arrives at the office on Tuesday, she finds Stephanie's lifeless body, shot once in the back of the head. Horrified that the burglar freed by their identity switch could have returned to kill Stephanie, the twins set out to solve her murder -- only to find themselves face to face with a killer who won't think twice about killing again.