In the 1840s to '50s America, both photography and embalming the dead came into standard practice. On the autism spectrum, Osborn Roche is a well-respected postmortem and battlefield photographer during the civil war. He and his nephew Ray travel with the Union army, photographing battlefields and doing portraits for the soldiers. 

When Osborn meets Lou, a young woman dressed as a man to work as an embalmer for the Union army, he is drawn to her beautiful, flickering, golden-brown irises, a condition known as nystagmus and also a condition Osborn's mother had. Only a day later, he sees that Lou is also on the autism spectrum as he is, and they become best friends. 

Lou works with her famous father Henry Cattell, a true historical man and embalmer of President Lincoln and his son Willy. From misinformation, Henry has strong feelings that his daughter should never marry or have children due to her condition. When he thinks their friendship may blossom into courtship, Henry threatens Osborn and takes Lou away. Having been picked on his whole life and never having a friend before, Osborn is broken without her, but goes on with Ray back to the Union army. 

By accidentally being in the middle of the battle of Corinth, he becomes a very famous photographer with a published book of it. He and Ray attend his war photography book signing, and Lou shows up in line with her uncle Ben. Ben explains that her father would have Lou sterilized by a surgical procedure and that he needs to get her away. Lou runs off with Osborn, his nephew Ray, and a runaway slave named Abigail, all heading back to their home in Pittsburgh, but are pursued by the Union army. 

Along the way, Osborn and Lou decide to marry to thwart Henry's claim. Henry convinces the authorities that Lou's condition is insanity and that she has no idea what she's doing. Lou and Osborn must escape again, but this time to Indian territory, a place they should never be found.