The Sixth Doctor is chasing a time anomaly in the London of 1986. Locating it in a sewer, he is able to better define it. It's a Time Wake — a literal wake in the time vortex that connects 1986 to 1720. It's caused by the use of primitive time travel technology. To close it, the Doctor has to stop whoever is using that technology. He decides the best way to do that is to follow the Time Wakes back to their source. He and Peri step through the wake — manifest as a circle of shimmering blue-white light —and end up in 1720.