The Doctor: Jodie Whittaker
Vinette Robinson – Rosa Parks
Joshua Bowman – Krasko
Trevor White – James Blake
Richard Lothian – Mr Steele
Gareth Marks – Police officer Mason
David Rubin – Raymond Parks
Ray Sesay – Martin Luther King
Aki Omoshaybi – Fred Gray
David Dukas – Elias Griffin Jr
Morgan Deare – Arthur
Jessica Claire Preddy – Waitress
Attempting to return to present day Sheffield, the Doctor ends up bringing her friends to Montgomery, Alabama in 1955 by accident. Before attempting to leave, the Doctor finds that there are traces of artron energy in the area from another time travel device. Deciding to investigate, the group learn that they have arrived the day before Rosa Parks refused to give up her seat as bus driver James F. Blake demanded on 1 December, effectively influencing the civil rights movement. Tracing the energy, the group locate a suitcase of equipment from the future, but are unable to learn more when its owner forces them to flee. The Doctor theorises they are attempting to change history by ensuring Parks never had to refuse her seat to Blake.
Leaving her friends to research everything behind the critical moment on 1 December, the Doctor returns to examine the suitcase's contents, before using it to shield herself from the owner. The Doctor soon learns that its owner, a rehabilitated murderer named Krasko, seeks to change Parks' life, since his neural implants prevent him from killing any living being. Unable to convince him to abandon his plan, despite destroying the vortex manipulator he used, she and her friends focus on thwarting his scheme by ensuring Parks refuses her seat to Blake at the appointed time, under the exact same conditions of the incident. Although Krasko works to counter their efforts, the group stay one step ahead of him, making certain to keep history on track.
While Ryan does his best to remove false notices at bus stops, he encounters Krasko blocking the bus route with a car. Ryan learns that the criminal's actions are motivated by his deeply racist views, and so uses his displacement device that he dropped to send him into the past to stop him completely. Removing the blockage, he and the others rejoin the Doctor on the bus as passengers. As the moment arrives, the Doctor realises they have become integral to events, and is forced to keep them aboard the bus. After witnessing Parks being arrested by the police for violating segregation laws, the group return to the TARDIS, learning that they have ensured she will go on to become a living icon for freedom.