Primary School No. 17 had always seemed like an ordinary place to Lena - until one grey Monday when a mystery entered her life. She was just sitting at the end of the corridor, staring out the window, when she felt a disturbing shiver down her spine. She had the feeling that someone was watching her, even though the corridor was empty. For several months, Lena had known she was different. When she concentrated hard... she was disappearing. She disappeared completely - her body dissolved into the air and she became invisible. No one knew about it, not even her best friend, Tosia. That day, her biology teacher called her up to the blackboard. Lena felt the familiar sting of anxiety. If only she could disappear... But she already knew that her power was out of control if she was too stressed. A thought flashed through her head: "What if I disappear now, in front of the whole class?" At the same moment, a scream rang out in the corridor. Lena looked out of the classroom door and saw a group of students gathered around the school lockers. One of them was holding a strange, glowing object in his hand. It was a watch which, legend said, had once belonged to a former pupil who had disappeared under mysterious circumstances years ago. Everyone froze, and Tosia looked towards Lena with terror in her eyes. Lena felt her heart beating faster. Then someone suddenly called out: "Someone was here and disappeared! I saw footprints that suddenly broke off!" The teachers began searching the corridors, and Lena felt her power begin to activate again. She looked around nervously, trying to figure out how to avoid the danger that suddenly seemed very real. Would someone discover that she was the one leaving these invisible footprints? And what is actually hidden in the mysterious watch? Lena crossed the threshold of the empty classroom, feeling the air thicken around her - at the exact moment when quick footsteps sounded and someone tugged on the door handle....