Like every year, Toby spends his summer vacation at his grandmothers and grandfathers place in Spain. Toby has no idea yet how memorable his vacation will turn out to be this time and spends some time at the beach until his friends arrive. Suddenly he hears the whining of a little dog coming from the bushes. He approaches it and meets Mister Blackjack, a very extraordinary dog. Toby adopts the little dog and takes good care of him. They play fetch, wander around the area and have a lot of fun together.
After his friends Max, Lewis, and Emma finally arrive, they all go to the beach to explore and discover a hidden cave entrance. Despite the prohibition sign with the warning, the childrens curiosity wins out and they enter the cave. They have no idea of the great danger and get lost in the labyrinth of the mysterious cave. And then suddenly something unexpected happens.