The Doctor and Romana visit Prague in the 30th century because a man named Kaj Jepperson has invented a time machine. He barely listens to them, complaining that his assistants, Frantiek Hrabal and Emilie Smetana, have been murdered so he has no time to answer questions. When they learn he is using zygma energy, they unsuccessfully try to talk him out of completing it.
As they explore the city looking for something to eat, they are accosted by a strange warped figure. They realise it is a future version of Jepperson. He was thrown backwards in time and has been waiting fourteen centuries for this moment — the moment that he remembers the Doctor and Romana visiting him. This Jepperson is the one that killed Hrabal and Smetana while trying to dissuade them from continuing their work with the younger Jepperson.
At Jepperson's lab, the older Jepperson tries to stop the machine but it is too late. The younger Jepperson realises what is happening and is dragged into the machine. The older Jepperson fades away while the younger one is sent back fourteen centuries into the past.
- Landmarks in Prague include Charles University and the Astronomical Clock.
- The Doctor hopes to eat a kolache.
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