Dred Scott, his wife and two children had voluntarily traveled alone almost 1200 miles down the Mississippi River from present day Minnesota, which was then free Wisconsin to meet his then master, Dr. John Emerson. The relationship was apparently a good one. When Dr. Emerson died, Scott offered to buy his freedom from the widow, but she refused. He then brought suit in the local circuit court of St. Louis, Missouri. The case was Scott vs. Emerson. Scott lost his original case, but the trial judge agreed with Scott's lawyer that the verdict had been based on hearsay evidence, and declared a mistrial. The case was retried in 1850 when a jury ruled in favor of Scott and his wife, determining that they were illegally kept as slaves while in Wisconsin territory where slavery was illegal. Mrs. Emerson appealed to the Missouri Supreme Court which overturned the verdict. The Missouri Supreme Court decision reversed a 28 year precedent.
A new case was brought in federal court on the basis of diversity of citizenship; because the Emerson Estate was probated by Ms. Emerson's brother after her death, Dr. John Sanford of New York. Scott lost again in Federal District Court, and the case was appealed to the U.S. Supreme Court. You know the rest of the story.