As The Red Badge of Courage opens, members of a newly recruited Union regiment are debating a fresh rumor: they are finally going to move out on the next day and engage the enemy. One young soldier, Henry Fleming, reflects on what will become of him when he gets to battle - namely, will he run or will he stand and fight bravely? He enlisted because he wants to be a hero, like the warriors of the Greek epics. His own mother, however, was not interested in such fanciful notions of bravery, and discouraged him from enlisting. When he tells her he's joined the army, she denies him a touching farewell scene and merely says if he finds himself in a situation where he will be killed or may do something wrong, he should go with his feelings.
He has not seen his foes yet, save for a conversation with one across a riverbank late one night. The veterans tell the young soldiers of gray, mad, rampaging hordes; but he does not trust the veracity of their tales. He does not care who he fights as long as he does not run away. He and his fellow troops – the tall soldier (Jim Conklin) and the loud soldier (named Wilson) – try to discern whether or not they will think of running. Both believe in themselves enough to say that they will fight as hard as they can, but neither goes as far to say that they definitely will not run.
The regiment does not move out on the rumored day, but soon thereafter. They march with other Union army regiments dressed in blue. Their untested nature is shown in the gleaming newness of their unscathed uniforms. The men run down roads carved out in the forest. During this time, Henry's thoughts are mixed and distracted. One minute he feels that he should have never enlisted and misses his home, but the next moment he feels the overwhelming urge to observe a battle. From a safe place on a hill, he watches a skirmish in quiet fascination but does not experience the desire to participate. After he sees his first corpse, he begins to suspect that his regiment is being lead to their slaughter, to be sacrificed to a red war god. He wants to tell his friends his theory, but is afraid they will laugh.
Soon, the regiment faces an actual conflict. Wilson, the loud soldier, is so certain he will die that he gives Henry a packet of letters to send to his family. As they line up to fight, rumors fly again about the state of their army. Smoke and noise from guns rise around them. Bullets and shells whistle towards them. A regiment in front, already engaging the enemy, is beaten and flees the battleground. The youth imagines they have been beaten by a monster. He resolves to get a view of this monster, even if he might very well flee himself. The regiment is soon engaged. They work feverishly, firing and reloading. The smoke chokes them and makes their eyes red. Henry is full of rage. Men fall occasionally around him. Soon, the enemy retreats and the men relax. Henry feels satisfied that he has overcome the trials of war.
However, the men have not rested for long when the Rebels attack again. They fight fiercely once more. Henry feels differently this time; he believes that the monster of war, a red and green dragon, will come through the gray smoke and swallow him. After a few men around him flee, the youth's own fear gets the better of him. He drops his weapon and runs from the battle. As runs through the forest, he is sure that the dragon is pursuing him and that those who fight are fools. However, as he finally stops near an officer, he learns that his regiment won the battle. He is thunderstruck. He realizes that he has done something very wrong, though he tries to justify his actions by convincing himself his superior powers of observation led him to conclude his regiment was in trouble and that fleeing was necessary. He imagines the insults he will have to bear when returning to camp and attempts to get as far away from them and the monster of war as possible.
Henry walks into a forest and the noises of the conflict gradually become fainter. He feels more at peace, as though his actions are supported by the natural world. However, as he walks, he encounters a corpse with a faded uniform. The glassy-eyed stare holds him for a moment in fear. Then the youth slowly turns away, creeping from the body until he is strong enough to run away as fast as possible.
He ambles through the forest and into the open. He finds a procession of wounded soldiers. They are suffering and moaning as they limp down the road. A tattered soldier, wounded twice, tries to talk to Henry about the battle and asks him where the youth has been shot. These questions stoke his embarrassment and guilt and Henry tries to run away in the crowd. He eventually runs into Jim Conklin, the tall soldier, wounded and near death. Henry tries to help him, but his friend will not allow him to. The tattered man comes up to assist as well, but Jim runs off into the fields, where he staggers and falls over dead. The tattered man tries to talk more with Henry, telling him stories of men he knows in the army and how he became wounded. Again, the man asks Henry where his wounds are located. The youth tells him to not bother him, and slips away from the man, leaving him blubbering and wandering about in the field.
As he continues on, Henry eventually encounters a retreating band of carts and horses. This makes him feel temporarily good; if the whole army is retreating, his flight will not be so conspicuous. He sees a column of troops come up the road. Inspired by their bravery, he gathers his will to fight but more thoughts soon crowd his head. He considers that he is low and guilty and knows his comrades will see him as a worm. These thoughts make him thirst and ache. He tries to justify his flight in his head, but his emotions betray him. He wishes he were dead.
Soon, the column comes running out of the grove into which they marched. All is chaos and pandemonium. Henry is shocked to see that these heroic figures are so quickly turned into scampering animals. He tries to stop one to ask him what happened, the man can only blubber. The annoyed and frantic man hits him on the head with his rifle. Henry is dazed and injured. He wanders in the dark until a kind man, the cheery soldier, helps him find his regiment.
There, no harsh words await him. Wilson and another soldier bandage his wound, which Henry claims is from a bullet. The others do not ask him questions, instead they kindly tell him to get attention and rest. When he awakes, he finds that his friend, Wilson, is not the loud soldier he once was – he takes special care of Henry, is reflective, and breaks up fights around him. The youth notices this change from irascibility to tranquility. However, he feels that he has a weapon against his friend in the packet of letters he gave in haste at the beginning of the battle the day before. Fearful of being discovered as a coward, Henry imagines he can use the packet to ward off shame or questions about his fear. Wilson was equally fearful as evidenced by the letters. However, Wilson sheepishly asks for the packet before Henry can use them against him.
The regiment moves from one embankment to another, glimpsing battle but not actually participating in it. The youth is now talkative, perhaps overly so. A sarcastic soldier cuts him down and later the young lieutenant tells him to stop talking and start fighting. The regiment does this soon enough. They are attacked by the Rebels and repel them. This battle, Henry fights as if he were crazed, shooting at them long after the battle is finished. This makes some of the men look at him with curiosity. Henry regards himself as a barbarian.
Soon, Wilson and Henry take an opportunity to get water for the regiment. After they search for a stream unsuccessfully, they encounter a general and his staff. In the midst of the conversation, they overhear that their regiment of "mule drivers" is going to charge the enemy, with perhaps many casualties. They return to their fellow soldiers with this news, but do not tell them that the general doubted that they would survive.
The charge begins. After a hesitation, the regiment runs with haste at the enemy. Many are shot in the process. Henry now feels that he sees things clearly. He and the other men go into a frenzy but eventually freeze. The lieutenant yells, screams, and curses at them to continue. Wilson breaks the spell by firing his rifle. Others follow his lead. Henry sees the flag of his regiment, which revives him. As his color sergeant is shot, he leaps for the flag, along with Wilson. The battle rages on, with Henry holding the flag aloft. The men dig in slightly, as their numbers diminish. Henry is full of rage. He is thinking little, only feeling his anger. The lieutenant and Henry are both trying to get the men to continue. Soon, the officer sees that the men in gray are trying to advance onto their position. Automatically, the regiment fires into them, causing the enemy to retreat. Satisfied, they go back to their lines.
When they return, they are greeted with jeers from the veterans and reprimands from the higher officers. They stopped short of an impressive charge, they learn. The men, who had been so proud of themselves, find that their efforts are not seen as sufficient, let alone brave. Soon, though, Wilson and Henry hear a story through one of their fellow soldiers that a colonel and lieutenant were discussing their particular prowess in battle. This fills their hearts with pride.
Soon, the battle is on again. The men in blue charge the men in gray once more. Again, the regiment finds itself in open territory, peppered by bullets. Henry is intent on standing upright, keeping the flag erect, though the men around him fall. Then the order comes to charge. As they approach the enemy lines, the opposing flag comes into view. Wilson leaps at it and grabs it from the hands of the wounded Rebel color sergeant. The men in blue are victorious, taking four Rebel prisoners of war, all very young and very human.
Henry, upon walking away with the regiment, first feels pride in his accomplishments of battle. Then he remembers his flight and his treatment of the tattered man, and guilt rises up in him again. He is concerned his mate will see it. However, he eventually lets this go, and now sees his previous thoughts on war and battle as silly. He has made it through the trials of battle, from the red and the black, and is changed into a man. The golden sunlight streams through the clouds as he marches with his regiment.