The Hamsterdactyls appear when the Turbo Toilet 2000 knocks the treehouse again and again and the eggs that they were made in cracked. They took a liking to Yesterday George and Yesterday Harold. However, they realized that the boys were in danger and destroyed the Turbo Toilet 2000.
When the effects of the 3d Hypno ring was and the super powers were extracted from Mr. Krupp, by Mr Meaner, the hero Captain Underpants was erased from him. Adult George and Adult Harold tried to contact their younger selves telepathically how ever their younger selves were in too deep of sleeping but the Hamsterdactyls responded and went to battle Mr Meaner putting a stop to his madness in the city to save Adult George and Harold. Later on the present time George and Harold took them to go back in time to meet their parents Crackers and Sulu. leaving the yesterday time of the past behind.