When the T.A.R.D.I.S. drops them in 1955 Montgomery, Alabama, The Doctor notices artron energy traces not related to the T.A.R.D.I.S. and goes out to investigate. Ryan and Yaz experience the racism rampant in the U.S. South of the 1950s. They meet Rosa Parks and realize the next day is the day she makes history.
They track the artron energy back to an alien ex-convict named Krasko who wants to prevent Rosa's influence on history from happening because he's just that racist. Because he's prevented from harming or killing anyone by a neural restrictor implanted in his brain, he has to disrupt historical events in non-violent ways. The team works out his plan to interfere with the fateful night of December 1, 1955, and takes action to correct all his changes.
Ryan eventually confronts Krasko and uses his own weapon against him to fling him into the past. In order to protect history from being changed, The Doctor and the Companions have to become part of Rosa's story, taking up seats on the bus the night she is asked to move for white passengers and witnessing her subsequent arrest.