William Brown, who wants to become a detective, writes to an agency and receives for $5 a tin badge. At the same time, a secret government formula for explosives is stolen from Lieutenant Musgrave, to whom Brown promises his aid. Meanwhile, Musgrave is hospitalized in delirium and utters the name "Wallace" to his sister, Barbara, who discovers that Wallace has hidden the formula in a flowerpot and follows him to his apartment. There, during a struggle, a book on flying alerts Brown. He accidentally discovers the secret formula, and Barbara escapes with the paper but is sidetracked on her way to a government agent. Following numerous complications and a long chase, Sherlock Brown succeeds in tracking down the conspirators, regaining the formula, and winning the heart of Barbara.