The Doctor and William visit a mobile city that is far in William's future in order to get fuel for the TARDIS. An insectoid named Fingal arranges a meeting for them with Eldric Bansal, who can get the fuel for them. However, Bansal is murdered during the meeting, and the inspector, suspecting the Doctor and William, gives them several hours to investigate and clear their names.
They keep encountering the name Boro Berikuka, part of Bansal's last words.
They first meet with Electra Montaigne, Bansal's ex-wife, who sends them on to Cardinal Xaluus Monfreit. En route, they get separated, and the Doctor meets with the Cardinal alone. The Cardinal tells him that Bansal had discovered something wrong done by the City Planning Authority a long time ago.
William wakes in a kind of cell with a screen. He tries to communicate with the screen, which admits responsibility for Bansal's death.
The Doctor, using the information given him by the Cardinal, meets with Unsil Girade, formerly of the City Planning Authority. Girade admits that the Planning Authority killed a boy named Boro Berikuka and used his brain pattern to run the city. Girade denies killing Bansal.
William teaches the screen to sing since it can't talk. He hums some music that he had found in the TARDIS.
The Doctor informs Girade that Boro is coming alive. They hear the city singing, and a time bubble opens, swallowing Girade.
The Doctor and William, who has been freed, meet up. The Doctor tells William that Boro killed Bansal, who had been planning to remove the city's autonomy.
When they leave the city, William asks that they return to two hundred years later to see how the city has fared. It is doing well.
- Sixth Doctor
- Eldric Bansal
- PR girl
- Electra Montaigne
- Cardinal Xaluus Monfreit
- Unsil Girade
- William expresses a wish to visit the Vikings.
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