If it hadn't been for the school competition, Lena, Igor and Maks would probably never have flown to Jupiter's moon. But the prize was special: a week at the 'Io-3' Research Station, the most modern scientific base in the Solar System. The station looked like a giant metallic spider perched on Io's yellow desert. Upon landing, the friends were greeted by a robot guide named Zeta. Zeta knew the answers to all questions and was always cheerful. On the first day, the children learnt the secrets of the local volcanoes and looked for signs of life under the ice crusts. However, at night Lena heard a mysterious sound coming from an inaccessible part of the station. It was quiet but distinct, as if someone was tapping on metal pipes. The next morning, she told Maks and Igor about it. They decided to check where the sound was coming from. Zeta strongly advised them against going to the closed section, explaining that the place was "under renovation". However, they couldn't stop thinking about it. Something must have been going on in there! In the afternoon, while Zeta was busy showing other visitors around, the three slipped into the forbidden section of the station. They noticed strange footsteps on the floor, as if someone was dragging something heavy there. The further they walked, the louder the sound became. Suddenly a door opened in front of them. Inside, amid flickering lights and scattered cables, someone or something was waiting for them. Lena took a breath and Igor squeezed Maks' shoulder. At the same moment, a strange, artificial voice rang out from within the darkness: "You shouldn't be here. If you want to return, you must first answer the riddle..."