On patrol, our two companions discover a young lady bathing in a pond. Cut hair, she managed to make herself look like a boy and enlisted in the northern army. Her goal? Search for her missing brother. Our friends will help her. Military life continues in an excruciating time. From cavalry charges to makeshift cantonments, from mud fights to patrols under rain deluges, they soon find themselves in the southern lines. Surprise! The young lady is actually a spy infiltrated who, thanks to Blutch and Chesterfield's involuntary help, has been able to gather a large amount of information that she will pass through with her friends. Our companions are taken prisoner. Will they manage to escape? What will the spy become? Will the information of her come to the enemy?