The Eight Hundred recounts a key moment in the Battle of Shanghai in 1937 during the Second Sino-Japanese War when 452 Chinese soldiers, led by Lieutenant Colonel Xie Jinyuan (Du Chun), are ordered to defend the fortress-like Sihang Warehouse against the much larger Japanese army. Slowly, the defenders' numbers are whittled down while repelling one enemy attack after another. Meanwhile, the siege is being watched by the initially dispassionate citizens of Shanghai's opulent International Settlement on the other side of Suzhou Creek. As the Chinese soldiers' situation becomes more desperate, it awakens a dormant patriotism that inspires the citizens to help them.
