Lieutenant Willy Fraser, formerly of the Royal Flying Corps, has been delegated the most dangerous job on the Western Front - a balloon observer hanging under a gasbag filled with explosive hydrogen, four thousand feet above the Ypres Salient, anchored by a slender cable. Swept across enemy lines after his balloon is damaged, Willy is hidden by Belgian farmers, whom he grows close to during his stay. With their aid, he manages to escape across the flooded delta at the English Channel and return to his duties. But once he's back in the air, spotting for artillery and under attack, Willy is forced to make an impossible decision that threatens the life of the woman he has come to love.