Cosmium is a gravity-repelling metal mined on Pluto and reduced to a substance called Gravitex. Nucleo, the Mad Martian, is suspected to be responsible of the theft of large amounts of the Gravitex. Following Nucleo to the planet Doba, Commodore Rex Clive of the Space Rangers and Stella Dawn of the STARS encounter a Lizardine sent by Nucleo to attack them. Rex kills the creature, which he thinks is the last of its kind.