This story has echoes that reverberate right back to 1964 with the Dalek Invasion of Earth. Susan, the Doctor's granddaughter, left him then, and stayed on an Earth ravaged by the Daleks to be with David Campbell.
In this story, the Doctor, still Grandfather to Susan, returns to England to see Susan, as he is concerned about what she may be involved in. Here, he meets also his great grandson, Alex, who is definitely getting mixed up in Earth politics - and more? Earth is slowly being rebuilt - technology is feared, those on the Moonbase are unable to communicate with Earth, and aliens are feared - anyone over 35 years of age still remembers the Dalek Invasion and what it was like. So Susan, while holding a respected position as regards the Earth Council, treads a fine line; particularly when it appears that aliens are approaching Earth again. But who are they, and what do they want?
This story is extremely intriguing in that we, as viewers and listeners of Doctor Who for many years, know so much more of the Doctor's adventures and where the time, for him, has gone, than Susan does. For Susan only twenty or so years have passed; for the Doctor so many people, places, times. So there is a touch of poignancy in the story that underlies the current action. It is a good `time and place' setter, if you will - it sets up stories for the Doctor to interact with his grand-daughter and great-grandson from now on, and it gives a nice sense of placement for the listener, hearing old familiar voices and giving a sense of familiarity to those of us who fondly remember the `classic' Doctor Who. Highly recommended.