Back in present-day England at last, the Doctor begins to power down the time bridge—but suddenly the power in UNIT HQ goes out, and Yates sees that the lights of London have changed outside the window. Realising what has happened, the Doctor stops before shutting down the time bridge completely. Salutua must have existed on a nexus point in time and space, and by passing through the bridge and interfering in the past the Doctor and his friends have changed history. While the time bridge was operating at full capacity the two timelines co-existed side by side, causing the spectral sightings which began on the opposite side of the world and have slowly been approaching the terminus of the time bridge. Since UNIT HQ's electrical generators no longer exist, the time bridge only has a limited amount of power, and when that runs out the time bubble preserving this corner of UNIT HQ will be wiped out of existence. The Doctor and his companions must find out how history has changed, return to the Constitution, and fix things before their timeline ceases to be.