Stefanos and his friends walk on the imposing walls that embrace the Great Castle, in the old town of Heraklion, searching for forgotten passages behind rusty doors. What role will the elderly Natalia Nikiforou play in their adventures, watching them from behind the curtains of her old mansion?

What is the relationship between the children and their younger brother and father, who disappeared in 1943? What will Stefanos' grandfather reveal that will turn everything upside down?

Will the children manage to solve the mystery?