The Second World War. A group of Royal Air Force officers led by William Ash (Jakub Reizer) is staying in Oflag XXI B, located in Szubin (Poland). The prisoners decide to organize a diversionary action, which is to be a daring escape through a tunnel, named after the main originator – the Asselin tunnel. They are helped in their activities by the Polish resistance movement, which includes a young photographer, Henryk Szalczyński (Michał Matela), who develops photos for forged documents.