In May 1940, as Hitler's armies tear through France, two battalions of Guards are rushed to Boulogne in a desperate attempt to hold back the German advance.

Lieutenant Jimmy Gale never expected to join them. Yet within hours he is leading a hastily assembled force of young sailors into a city already descending into chaos. Poorly trained for land warfare and armed with little more than rifles, they must face tanks, machine guns, mortar fire and relentless attacks from the air. Their orders are simple: hold the line and bring as many men home as they can.

More than seventy years later, Jimmy's son sets out to uncover what really happened during those violent hours, and why his father received one of the Royal Navy's highest awards for gallantry.

Based on real people and extraordinary events, One Bloody Day in May is a gripping story of courage under fire, ordinary men called upon to do the impossible, and a son's determination to understand the quiet, cautious father who once led his men into hell.