Karolina sat on the windowsill and watched the sun paint shadows over the old school playground. She counted the cat's paws imprinted in the concrete for the hundredth time, but she couldn't stop glancing at the school's roof. There, on the very edge, something glinted. "Tom, come on!" - she called out to her little brother, who was just finishing laying out the dominoes. Tom ran up and raised his glasses to his nose. They were different from the other children - this they had known for a long time. Carol could disappear whenever she concentrated very hard, and Tom he could stop time for a few seconds. Since they discovered these powers, they formed their own Invisible Club, meeting on the roof of the school. There they felt safe and free - between chimneys, aerials and the rustling wings of pigeons. They were the only ones who knew how to get quietly onto the roof through the watchman's window. This afternoon, however, was different. Karolina noticed something completely new. A shadow had appeared on the roof, where usually only old newspapers and a few drinks cans lay. The shadow had the shape of a figure that moved very slowly, almost imperceptibly. "What is it?" - Tom whispered. Carol took a deep breath and held out her hand to stop her brother from making a noise. Slowly - on tiptoe so as not to make a noise - they opened the heavy attic window and stepped out onto the hot sheets of the roof. The figure in the shadows suddenly turned around. Its eyes flashed with blue light, as if it could see everything - even the things that tried to hide in the darkness. Karolina felt energy begin to tingle in her hands, as if invisibility wanted to break free on its own. Tom was almost out of breath. Suddenly the figure took a step forward, directly towards them, and then the roof beneath their feet trembled. Was it possible that someone else had discovered how to get onto the roof? And was he as unusual as they were? At that moment, a shrill meow sounded from the neighbouring chimney and a shadow began to dissolve, squeezing between the tiles....