Medical student Martha Jones helps the Doctor thwart an army that has transplanted a London hospital onto the moon.
Harry Saxon becomes Prime Minister, but has dark ambitions.
With The Doctor imprisoned by The Master, Martha endeavors to save the world.
Mark plots to sabotage Jeff and Sophie's corporate trip to Aberdeen, and Jeremy struggles to contain his jealousy when Nancy talks to other men.
William is working overtime handling the tragic deaths of three teenagers. Mary has kicked Molly out for her meddling, and she dashes William’s hopes when she tells him to slow down.