Henry Fleming is a teenager with romantic notions about the glories of war. He enlists in the Union army and quickly discovers sides of himself he never knew existed. The horrors, boredom, and complete injustice of war bring out all of Henry’s worst (and occasionally best) tendencies.
Initially, Henry fears that he will run like a coward when faced with his first battle. He’s been in the army for a while now but hasn’t seen any action yet. Talking with the other men, he tries to get them to admit that they are scared as well. No one wants to say as much; they all seem perfect examples of fearless men, which leaves Henry feeling even worse about his own apprehension. Shortly before his first battle, he sees his first dead body, a gruesome corpse.
Meanwhile we meet two men, Jim Conklin or "the Tall Soldier" whom Henry has known for years, and Wilson or "the Loud Soldier." Wilson, afraid that he will die in battle, gives Henry a packet of letters to deliver to his family after the war. When the fighting finally starts, Henry doesn’t do too badly. However, when a second round of fighting begins after a brief lull, Henry is terrified and heads for the hills. Afterwards, he tries to rationalize his decision (to himself) by claiming it was simply a survival instinct. He oscillates between a feeling of superiority and one of crippling guilt.
As Henry heads back toward the sound of battle, he encounters a group of wounded men leaving the scuffle. Seeing their bloody injuries, Henry wishes that he, too, had a red badge of courage. One of the wounded men, a Tattered Soldier, keeps asking where Henry’s injury is, which of course makes our protagonist uncomfortable and nervous that he will be found out. Henry then bumps into Jim Conklin, who dies a rather horrible death in front of him. Henry runs away, encounters another group of men, gets whacked on the head with a rifle butt by a rather freaked out member of his own army, and amazingly ends up back with his own 304th regiment. To his relief, no one accuses him of running away. It seems that everyone got separated in the confusion of the battle. Seeing his head injury, the men assume a bullet grazed him.
Henry then encounters Wilson, who asks sheepishly for his packet of letters back, making Henry feel superior to him. Henry milks this superiority for all it’s worth to justify his guilt over running from battle. He loudly disses the general’s tactics and blames his strategy, rather than his regiment’s quality of fighting, for the losses suffered that day.
Yet another battle begins, but this time, Henry is ready. He fights wildly and afterwards is praised by his lieutenant for his actions. However, in a lull between battles, he and Wilson overhear a general referring to their regiment as "mule drivers" and preparing to sacrifice them at the front line in the next scuffle. Henry wants more than ever to prove himself. In the next battle, he and Wilson see the Union flag bearer fall. They rush forward, take up the flag, and rally their comrades to fight. Afterwards, the superior officers praise this courageous action. In the novel’s final battle, Henry captures the Confederate flag as well and helps lead his regiment to victory.
When the battle is over, Henry reflects on his actions. He decides to accept his earlier, cowardly actions rather than pretend they never happened. He sees himself as a real man who has survived "the red sickness of battle" (24.33).