How strangely the new passenger of the boat on which the members of the Detective Club are traveling - Holly, Tracy, Belinda - behaves strangely. Why did she pretend to break her leg? Why did she persuade the inseparable trinity to sail along a dark and abandoned canal? Why does he flinch at an innocent question about his parents? And finally, why did she... give a false name?! Suspecting something was wrong, her friends decide to follow her on her heels. This is where the strange girl reveals the whole truth...