The Doctor, Vicki and Steven visit Charles University in Prague for its one-thousandth anniversary. They are welcomed by Professor Fox and several graduate students, including Jane Childress, who is wearing a necklace that Vicki admires, but Jane won't let her look at it.
A monkey appears out of nowhere, and Jane coaxes it to come to her with some fruit from her pocket. She offers to take the monkey to the tower of the Astronomical Clock so the caretaker, Jimson, can look after it. The Doctor sends Steven to accompany her.
Inside the clock tower, Steven finds Jimson's empty uniform, and Jane hits him over the head with a piece of wood.
Meanwhile, the Doctor is suspicious of Jane's actions and orders Vicki back to the TARDIS. When the clock strikes, the Doctor realises something is happening. Everyone starts to change, "devolving" into monkeys, gorillas or apes (with the exception of the Doctor, who devolves into something with tentacles). The Doctor retains enough consciousness to head to the clock tower, where Jane, seeing him, starts to boast.
She has long been ignored over her fellow, younger, graduate students, and she has built a "devolving" machine as revenge. Soon, she will be the smartest person on the planet. What's left of the Doctor destroys her machine, but she has a handheld device. She points it at the Doctor, but the monkey that used to be Jimson grabs it from her. At first, she thinks she's safe, as her necklace will protect her, but the Doctor reveals that he has taken her necklace. Jimson points the device at her and presses a button, and she slowly devolves into a flash of light. The Doctor turns off the device.
Later, everyone has returned to normal. The Doctor explains to his companions what happened, telling them that his people evolved over a much longer time period than humans, so he was able to retain consciousness long enough to stop Jane.
- Vicki mentions the Charles Bridge.
- The Astronomical Clock was built by Mikulas of Kadan and Jan Ondrejuv.
- Johannes Kepler and Tycho Brahe studied at Charles University.
- Albert Einstein was a professor at Charles University.
- Champagne is served at the celebration.
to be added