The Mostly True Adventures of Homer P. Figg by bestselling novelist Rodman Philbrick is the story of a young boy who runs away to find his brother, a young boy who has been sold into the army during the Civil War. In this novel, Homer has many adventures as he follows his brother to the Battle of Gettysburg. Homer meets a Quaker who is smuggling slaves out of the country, a preacher who is easily tricked by a con woman and her accomplice, and a medicine show owner who turns out to be a spy for the Confederates. In the end, Homer saves his brother from sure death only to place himself in mortal danger. The Mostly True Adventures of Homer P. Figg is a funny novel that presents true history to readers in a way that is both eye-opening, touching, and provides for a good laugh.

Homer P. Figg and his brother, Harold, live with their uncle. Their uncle is a cruel man who makes the children live in the barn. One day Homer takes a crust of bread from the slop meant for the pigs, causing the uncle to become angry. Harold defends his brother by pushing the uncle into the dirt. A short time later, a group of men come and force Harold to take an oath that forces him to fight in the Union Army during the Civil War. Later, Homer overhears the men talking about how the oath was not legal.

Homer runs away with the intention of telling his brother he does not have to fight in the army. However, on the first nigh,t Homer runs into a couple of men who have taken a black man captive and plan to sell him and the slaves he has been leading to safety in Canada back into slavery. However, they need Homer's help finding the slaves. Homer is sent to the home of Mr. Brewster to find out where he is hiding the slaves. However, Mr. Brewster has figured out what is happening and tells Homer he knows. Then Mr. Brewster shows Homer where the slaves are and trusts him to decide what to do.

Homer lies to the criminals and leads them to Mr. Brewster's mines, where they are ambushed by the conductor who is supposed to lead the slaves to freedom. The slaves are saved. Mr. Brewster offers to adopt Homer, but Homer insists on continuing his search for his brother. Mr. Brewster sends a new preacher with Homer named Mr. Willow, who has been entrusted with enough money to buy back Harold should it be necessary.

Before they even board the ship, Mr. Willow is spotted by a couple of con people. The woman seduces Mr. Willow and convinces him to give her the money Mr. Brewster has handed over for Harold. When Homer protests, he is knocked out and placed in a pig crate. This is where Homer is found some time later by Professor Fleabottom, the owner of a medicine show.

Professor Fleabottom takes Homer into his show and makes him the pig boy. After the first show, however, Homer learns that Professor Fleabottom is selling whiskey to the army soldiers. Professor Fleabottom explains that these men are about to die and they deserve to have a few moments of relaxation first.

While running from some angry army officers, the medicine show runs into a man with a hot air balloon. As they enjoy breakfast with this strange man, a group of military officers come up on them and accuse Professor Fleabottom of spying for the Confederates. As they attempt to arrest everyone, Homer gets away in the hot air balloon.

Homer crash lands in Confederate territory when the balloon gets a tear in it. As he waits in a barn stall, Homer learns that the Battle of Gettysburg is just beginning. When a call to arms takes away the soldiers guarding him, Homer steals a horse and rides to the Union side of the battle. There Homer begins searching for his brother. Homer runs into Mr. Willow, who has joined the army to make amends for allowing himself to be swindled.

Homer finds Harold only to learn he has been arrested for cowardice. Harold's regiment is called to arms and Colonel Chamberland frees the prisoners so that they might fight as well. Harold fights bravely as Homer brings the soldiers ammunition. When they run out of ammunition, the men fix bayonets and charge the enemy. Homer tries to stop Harold when he takes over the regimental flag. To stop him, Homer shoots Harold in the leg. When Harold falls, Homer takes up the flag.

After the battle, Harold is released from the army and he and Homer live on their own for several years until Mr. Brewster finds them. Harold recovers from his injury only to get an infection that causes him to lose his leg. Despite this, the two boys live happily with Mr. Brewster.