It's young Roger Marlow's birthday. Not only has his father, Captain Rod Marlow, returned from a deep space mission, he's also received an unusual gift. On a world from which the Daleks have had to retreat, Captain Marlow has found an orbitus, a robotic "pet" that can do practically anything. Roger gives his new toy a try and finds it supremely useful. It will do his homework, tell him a story, clean up his room — anything he tells it to do.