A group of Bulgarian cavalrymen, led by the daring Lieutenant Sergius, attacks a Serbian cannon commanded by the Swiss captain Bluntschli.

This seemingly suicidal attack unexpectedly ends in a Bulgarian victory because the cannon is supplied with the wrong ammunition and cannot be fired.

Captain Bluntschli, forced to flee on foot, seeks refuge in the house of the Bulgarian Petkoff family, whose daughter Raina is engaged to his adversary Sergius.

Raina, initially idealistic about Sergius's "great victory," becomes disillusioned after sheltering Bluntschli, realizing the harsh realities of war.

Bluntschli reappears, warmly welcomed by Major Petkoff, who admires his organizational skills and asks him to handle calculations for supplying the returning Bulgarian troops.