The Doomsday device is a device that Atomic Roger went to retrieve, he then returned to deliver it to DeGill. Not knowing of the events in No Space Like Home, Roger arrived at the remains of Galactic Guardian Headquarters. In his frustration, he kicked the device, accidentally (unknown to him) activating it.
Arriving on Earth to ask Betty about the new location of Headquarters, the device was separated from him and Purrsy got his claws on it and thought it was some kind of giant cat toy. Betty, Roger, and her team united to find the device before it went off. Betty was able to find the device and Sparky and X-5 teleported it to an uninhabited beach planet before it went off, the same planet, which unfortunatley, Maximus I.Q. and Minimus were vacationing at. (Roger, Where Are You?)