While Nancy's dad is away at a conference in Washington, DC, she invites Bess and George over for a slumber party. The girls are having a great time until Nancy gets a call from a number she doesn't recognize. Her dad never showed up for his panel and he isn't in his room. No one's seen him since the night before, and he isn't answering his phone.

Worried, Nancy and the gang hop the next flight to DC to investigate. The girls scour the hotel for clues with little luck until Nancy finds her dad's cell phone in the hotel basement. She's pretty sure he left her a message on the home screen, if only she could figure out what it means.

The hunt takes them across the US capital, retracing Mr. Drew's steps to figure out what went wrong or who might be out to do him harm. If they don't solve this mystery, it may cost Mr. Drew his life.