The TARDIS arrives in Montgomery, Alabama on November 30th, 1955. The Doctor detects artron energy emissions, and becomes concerned that someone is trying to interfere with established history. December 1st, 1955 is the date of a landmark event in the history of American civil rights: seamstress Rosa Parks' refusal to give up her seat on a segregated bus to a white person. But even as the four friends begin to unravel the machinations of the time-travelling ciminal Krasko, they must also contend with the era's deeply-embedded racism -- a problem that even the Doctor can do little to confront.