Dred Scott was forced to be a slave in a non-slave state by an army surgeon named John Emerson. When Emerson died in 1843. He had became a free man and had decided to sue Emerson's widow for being a slave in a free state. After defeat in state court. He had brought the issue to the supreme court.
The supreme court ruled in favor of the slave owner. The slave owner sold Dred Scott to the Blow family who released him shortly after.