“21 Brothers” narrates the experiences of the Canadian 21st Battalion as they gear up for the Battle of Courcellette during World War I. The film unfolds in real time, focusing on Sgt. Reid, who is tasked with preparing his troops for the imminent conflict. Alongside this, Sgt. Reid faces the everyday challenges of trench life, such as managing his soldiers’ injuries, handling supply shortages, and addressing the presence of a young recruit who has just been deployed to the front lines.