After a close call, Mr. Drew insists that if Nancy's going to be an amateur detective, she needs to be able to defend herself. So Nancy, Bess, and George decide to check out a Jiu-Jitsu class at Iron Dragon MMA. The technique is hard, but before she knows it, Nancy's having a lot of fun.

And then, just as class ends, the students are shaken by a disturbing sight--someone's left a dead rat on the front desk and spray-painted "traitor" on the wall. With a big competition coming up soon, is a rival academy trying to stir up trouble to throw the Iron Dragon team off their game?

Nancy agrees to help the team get to the truth, but as her investigation takes some unexpected and increasingly dangerous turns, has River Heights's finest sleuth been outmatched?