Jessica and Elizabeth Wakefield's father teaches them a secret language called Ithig. All their friends want to know the secret too, but the twins have made a pact with their father not to give it away. Elizabeth, at least, intends to keep her promise. But Jessica can't help herself—she teaches her best friend, Lila Fowler, the secret language. That means that soon the whole school will know Ithig.
Now the class decides to use the language against the one teacher they don't like - Ms. McDonald. They will speak only Ithig in class the day the school supervisor comes to review her. At first, Elizabeth is angry at Jessica for breaking her promise about Ithig—but now Elizabeth wonders: should she break her own promise and warn Ms. McDonald?