Earl Pilcher Jr. runs an equipment rental outfit in Arkansas, lives with his wife and kids and parents, and rarely takes off his gimme cap. His mother dies, leaving a letter explaining he's not her natural son, but the son of a Black woman who died in childbirth; plus, he has a half brother Ray, in Chicago, she wants him to visit. Earl makes the trip, initially receiving a cold welcome from Ray and Ray's son, Virgil. His birth mother's sister, Aunt T., an aged and blind matriarch, takes Earl in tow and insists that the family open up to him.

The story is about an aging cowboy and his nephew who transport 500 horses from Oregon to Wyoming to sell them to the British Army. Along the way, their simple horse drive is complicated when they rescue five Chinese girls from a slave trader, saving them from a life of prostitution and indentured servitude. Compelled to do the right thing, they take the girls with them as they continue their perilous trek across the frontier, followed by a vicious gang of killers sent by the whorehouse madam who originally paid for the girls.

Broken Trail weaves together two historical events: the British buying horses in the American West in the late 19th century and Chinese women being transported from the West Coast to the interior to serve as prostitutes.