Catherine is trapped, living moments of her life over and over. She can choose which moments she relives, but she can't alter them, no matter how hard she tries.
The TARDIS materialises in a corridor, and the crew deduces that it's an abandoned science base. They enter a room and both the Doctor and Nyssa recognise a cyclotron. While they examine it, Tegan tries to amuse herself. The Doctor is puzzled by some strange readings.
Particles seem to be appearing out of nothing and then disappearing. The Doctor says they are travelling backwards and forwards in time, but Nyssa corrects him and says they are only travelling forwards.
The Doctor and Nyssa conclude there is a consciousness in the cyclotron. Tegan starts talking, and repeats herself several times. She says she is alone and deserted. The Doctor and Nyssa realise it isn't her; the Doctor brings her back and begins to work out equations.
The Doctor figures out an entrance point and jumps into the cyclotron, after giving Nyssa coordinates for the TARDIS to meet him at an exit point.
Catherine jumped into the cyclotron when one of the team members on Hardy Base, Frank Paxton, attacked the others. Catherine escaped by jumping into the cyclotron.
The Doctor finds her. She is aware of him, but keeps jumping to different moments, followed by the Doctor. Now that the Doctor is with her, she has the ability to change things.
Meanwhile, the TARDIS is having trouble getting to the coordinates set by the Doctor. Nyssa thinks the universe is coming to an end.
The Doctor is having trouble convincing Catherine to leave the cyclotron. When she changes her history it causes damage to time. She isn't convinced when he tells her that the universe won't last much longer. The Doctor places himself in front of a moving vehicle, and at the last minute Catherine takes his hand and saves him.
They come out of the cyclotron, to the surprise of Paxton. Catherine hits Paxton with his own saxophone, then they enter the TARDIS, which has arrived safely at this time.
The Doctor brings Catherine back to Earth, ten months before she is scheduled to come home from Hardy Base. Her two boyfriends, James and Alec, are extremely surprised to see her. Nearby, the Doctor and his friends watch in amusement.
- Tegan draws a kangaroo and two Cybermen in the dust.
- Nyssa mentions the Traken Union.
- The Doctor drinks orange juice.
- The Doctor tries to find the section on universal warping in the TARDIS manual, but it has been ripped out.
- The stories in Decalog are linked by the story Playback, in which psychic Silverman examines various items in the stranger's pockets. For this story, he examines a saxophone mouthpiece.
- The Doctor and Tegan are still arguing about Adric's death and the former's refusal to go back in time and save him. (TV: Earthshock)