On the riverside quay of the enormous container port of Lisbon two old decaying sky-blue painted buildings are located directly opposite to each other. One is closed and abandoned, the other inhabited by the two bright and inquisitive girls Ana and Matilde. Watching the closed building every single day and night the sisters get more and more suspicious. Strange phenomenons and movements behind the thick blue walls increase their curiosity. How is possible what their children`s eyes are witnessing? Who are those mysterious neighbours? How to discover old house`s secret?