The Doctor and Companions had, in the previous episode, acquired a gizmo. A Time-Space Visualizer. This was one of the prizes in the space museum, and at the end of that series they showed the natives of that planet destroying all the gizmos in the museum. The Doctor said, uh, wait a minute, would you mind if I take that gizmo with me? The Time-Space Visualiser is a television-like thingymajob which can pick up on any event in the whole of time and space.