Lan was a Groog and a soldier. He was the younger but taller one of the two Groogs that visited Mary's and Bill's house after their space ship crashed on Earth.
At first it seemed as if Lan and his friend were attacking Mary and Bill. But it turned out that they tried to help them fighting some enemies that they couldn't see. The enemies were the Eleventh Doctor and Amy Pond, who were inan other dimension and tried to contact Rory Williams, Bill and Mary. Amy and the Doctor thought Lan and his friend were the enemies.
After Amy and the Doctor returned from their dimension, Bill and Mary invited Amy, Rory, the Doctor, Lan and his friend to breakfast and everyone accepted the invitation. The space ship of the Groog only needed a few repairs and the Groog would soon be able to use it again. (AUDIO: The Empty House)