Gemino was a planet inhabited by a humanoid race. It was devastated in a war with its twin planet, Gemina. During the war, all supplies and knowledge were locked away in the Vault of Plenty, which was guarded by a dragon. Some of the survivors of the war tried to gain access to the Vault, but they all died in the attempt.
Some time after the war, the First Doctor, John and Gillian visited Gemino, landing near the ruins of Gemino City. While exploring, they discovered a dog and followed it out of the city to the dog's owner, who told them about the Vault. Despite his warnings about the dangers, the Doctor and his grandchildren decided to open the Vault themselves. They correctly guessed the answers to the puzzles needed to gain access to the Vault, allowing the survivors to get their supplies. (COMIC: The Secret of Gemino)