And once again, it’s vacation—and once again the Five Find-Outers are in for adventures! After all, how could there not be adventures when the kids have moved in next to a half-ruined old castle? And sure enough: one day, someone’s face appears in the window of its tall tower. Who is it? A ghost? Or some unfortunate captive whom, for reasons unknown, are hiding in the ruins forgotten by everyone? Perhaps one of those mysteriously vanished scientists, about whom newspapers have written, is here? How can this secret be uncovered? Very simply: you need to see everything with your own eyes! And in the moonlit night, the friends make their way into the castle through a secret passage…