In 1955, moments after witnessing Emmett "Doc" Brown disappear in his DeLorean, Marty McFly learns that Doc was sent to 1885. Marty comes across a tombstone with Doc's name, with the inscription stating that Doc was shot by Biff Tannen's great-grandfather, Buford "Mad Dog" Tannen, six days after writing the letter. Despite the letter's warnings, Marty travels back to 1885 to save Doc and return once again to 1985, but matters are complicated when Doc falls in love with Clara Clayton (Steenburgen).

Source: Wikipedia