Harry's cover teacher is acting weird. On Christmas day there is a present for him from the Doctor. Harry goes to town and meets a man who gives him the key to his present. Harry also meets a women named Martha, who gives him a bottle of Aroma, a mixture that can stop the aliens. Harry sprays Aroma on an alien. He unlocks the TARDIS and sees an alien battle. Harry, Martha and the Doctor have to defeat Harry's cover teacher, who is really an alien with a cow's head and snakes coming out where her eyes are supposed to be.