It was summer on a quiet river in the town. Kaja and Philip were launching their bark boats. Suddenly something flashed by the bank, like a small fish. It wasn't a fish, but a tiny metal key. The key was lying in the grass, still wet from the water. Kaja picked it up carefully and rubbed it with her shirt. On the key was an anchor mark and an arrow. The arrow pointed to the old bridge on the other side. "Maybe it's a clue?" whispered Philip, until his voice trembled. They followed the path past reeds and an old willow tree. Under the bridge they found a box among the roots and stones. The lock fitted the key perfectly at once. Kaja slid the key in and heard a quiet click. Suddenly the grass rustled and the river became quiet. A whisper came from the bushes: "Don't open ... not yet." Kaja and Philip looked at each other, and the lid vibrated.