When a local schoolgirl named Elizabeth is blamed for stealing money from her teacher's desk, she does the unthinkable—she runs away from home in the middle of the night! With the police completely baffled and Elizabeth's family frantic with worry, Peter, Janet, Jack, Colin, George, Pam, Barbara, and Scamper the dog immediately launch a massive search. Armed only with the clue that she was last seen wearing a distinctive blue coat, the Seven comb the neighborhood looking for a hiding place.

But this time, the Secret Seven are not the only detectives on the case! Jack's mischievous younger sister, Susie, and her annoying friend Binkie have formed their own rival detective agency, and they are determined to find the missing girl first to make the Seven look foolish. Plunged into a fierce, high-stakes competition full of false leads and wild goose chases, the Secret Seven must outsmart Susie, track down the terrified runaway, and uncover the real thief to clear Elizabeth's name!