After Artaxerxes picks up Rover's yellow ball, Rover demands it back without saying "please". Rover bites Artaxerxes trousers when he continues to refuse to return the ball; in retaliation, Artaxerxes turns Rover into a toy dog and transports Rover to a toy shop.:1-2 Although Rover visits the Man-in-the-Moon with the help of Mew, and seeks the help of the sand-sorcerer Psamathos Psamathides, the great whale Uin takes Roverandom (as he is now known) to ask Artaxerxes to return him to being a normal dog.
Artaxerxes is living at the bottom of the Deep Blue Sea and is married to the daughter of the Mer-king. As a wizard, he has been appointed Pacific and Atlantic Magician and is known as PAM. All the woes of the mer-people are blamed on PAM, including the emergence of a Sea-serpent. Because he is so deeply unpopular, PAM leaves with his wife and Roverandom: he takes Roverandom with him as the dog continues to be polite and show faith in his magical abilities.:55 ff
Back on land, Artaxerxes turns Roverandom back into a real dog. Wearing a purple velvet suit, Artaxerxes sets up a chocolate and cigarette shop by the beach (selling Pam's Rock), whilst his wife gave swimming lessons.:83-7
Artaxerxes is the name of five Kings of Persia.