During the initial attack of the Black Cross Army against the Japanese EAGLE bases, Poison Gas Mask attacked the Kansai branch, killing everyone with Kenji Asuka the only survivor of the attack.
Poison Gas Mask's scheme is to use an underground gas factory to create his poison gas for hurting people. Kaijo and Asuka discover the factory but the Masked Monster avoid their attack outside one slight hit from Akarenger's Red Bute that hits his helmet, using that as an excuse to further develop his "immortality". Asuka becomes obsessed in defeating Poison Gas Mask, who further riles him up by attacking the spies and then sneaking into Snack Gon to investigate. Midorenger challenges him and loses due to his immortality ability, torturing him until he forces the other Gorenger to save him. In a final showdown, Asuka discovers the trick to Poison Gas Mask's ability, flipping nibly until he finally hits him from behind with a Midorang. Disoriented, Poison Gas Mask is easily dispatched by a Gorenger Storm.
Modus and Arsenal
He wears a European military uniform and a gas mask. He uses the gas mask to shoot gas to asphyxiate people while shooting them with a rocket he attaches to his wrist. He also concentrates himself in developing the technology in his mask to avoid any and all attacks, claiming himself to be "immortal", but really is using teleportation to avoid all direct attacks.