The sunny wind blew Martha's hair away as she walked along the seashore, wading barefoot in the sand. Next to her, Bartek and his younger sister Zosia were searching for shells. Suddenly, Bartek stopped abruptly. - Look! - he called out, picking something up from the sand. It was an old glass bottle. Inside, like a fairy tale, was a rolled up, yellowed piece of paper. Zosia, excited, helped Bartek take out the map. The sheet had the outline of an island painted on it, with strange symbols: stars, arrows and a big question mark in the middle. Beneath the map was an indistinct inscription: "Courage leads where no one dares to go". Martha looked at her friends with a glint in her eye. - What if it was a real treasure map? Bartek smiled broadly. - I think we're in for an adventure! The next day, with backpacks full of torches, binoculars and sandwiches, they set off towards the place marked on the map. The road led through a dense forest, where trees whispered mysterious stories and the rustling of animals could be heard every now and then. Zosia noticed paw prints on the muddy path. Eventually, they came to a small clearing where another sign from the map lay under the roots of an old oak tree: a stone circle. In the middle of the circle lay a shell, the same as the one in the bottle. - 'I think we need to do something,' Bartek said in a whisper. At that moment there was a deafening sound and the ground beneath their feet began to tremble gently....