Linda Grainger and her grandfather Edward Grainger are visiting Edward's old friend, Commander Guy Oliver, and are taking a tour of his submarine. Oliver receives orders to investigate a crashed ship in the ocean, and there is no time to bring Linda and Edward back home first. Edward offers his help, which Oliver accepts.
Meanwhile, the Doctor and Susan wake up to find themselves on a huge spaceship. They remember that they had been in orbit above Earth, but they were about to crash into another spacecraft. The captain of the ship, a Slarvian, welcomes them and says that he rescued them and is powering up their ship for them. When he learns they are not human, he treats them as guests.
As the sub arrives at the crashed ship, Lieutenant Brandis pulls a gun on the others and orders them to put in the coordinates that he gives them.
The Slarvian explains to the Doctor that he is an advance scout for his people, who are coming to Earth to conquer it. In addition, the Slarvian Queen must finish the gestation process with her eggs, which are in the crashed ship in the ocean. He tells the Doctor that he has a human agent working for him.
Brandis contacts the Slarvian ship. The Slarvians see Linda and tell the Doctor that their Queen must feed on a human female, and Linda has been chosen because the only other available female is Susan, who is not human. The sub begins to connect to the crashed ship, but the Doctor has a plan. If he can get to the TARDIS before the airlock is opened between the sub and the ship, he can drag the Slarvian ships with him, away from Earth.
Susan surreptitiously tells Linda that they must close the airlock. She and the Doctor run for the TARDIS, but it must power up first. On the sub, Linda distracts Brandis by thanking him for the tour of the submarine. The plan works, and the TARDIS is able to take the Slarvians away from Earth before the airlock is opened. Brandis recovers from the Slarvian control but has no memory of what happened. Edward downplays the whole experience, mentioning training exercises and pressurised conditions.
Back at home, Linda asks her mother for the old photo albums of her grandfather.
- First Doctor
- Susan Foreman
- Edward Grainger
- Linda Grainger
- Commander Guy Oliver
- Lieutenant Brandis
- Slarvian captain
- Linda Grainger's mother
- Susan wants to see the Great Wall of China from space. In attempting to do so, she observes Southeast Asia on the TARDIS scanner, however is unable to spot the wall.
- Edward tells Linda that he was named after Edward VII, who was King in 1906.
- Winston Churchill was Prime Minister of the United Kingdom at the age of eighty.
- The stories in Short Trips: The Centenarian revolve around Edward Grainger, a man who has met the Doctor more than a dozen times over the course of his life.