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. 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.