The sun was slowly setting over the small seaside town when Lena, Michal, Zuza and Philip gathered on the old breakwater. It was their last week of holiday together and they all felt that something unusual must be happening. Lena, curious and determined, swiped a file of yellowed pages she had found in her uncle's bookcase. - 'Look at this,' she said, unfolding the pages on the stones. - This is no ordinary letter. On one of the sheets was a drawing of a lighthouse, and underneath it a mysterious inscription: "When the light blinks three times at night, look for the key under the fifth step of the stairs." - Do you think this is some old joke? - Michael asked sceptically, but a spark of curiosity also flashed in his eyes. - 'If it's a joke, it's a very sophisticated one,' muttered Zuza, known for solving puzzles. Filip was already preparing his torch and phone, as he was the only one who always had everything at hand. They decided to act the same night the lantern would be automatically activated. They passed the sleepy town, the wild bushes and climbed the hill where the old lighthouse stood, menacingly outlined against the orange sky. As they reached the closed door, they heard the regular murmur of a mechanism inside. Suddenly, the lantern light flicked three times. - 'I think that's our sign,' whispered Lena. In silence they moved up the stairs. Michal's heart was pounding like a hammer and Filip glanced nervously at his phone every now and then. Under the fifth step, Lena found a small metal flap. When they opened it together, they took out a wax-wrapped scrap of map and... a small, rusty casket. Before they had time to open it, there was a quiet clatter from behind the wall, as if someone had moved among the shadows. Everyone froze, clutching the first clue in their hands. Were they really alone in the lighthouse?