“Why is Japan continually being attacked by giant monsters?” a beleaguered official asks in 1999’s “Gamera 3: Revenge of Iris,” which will screen this Friday as part of the Japan Society’s Otaku Cinema Slam! Series, celebrating the 40-year legacy of Gamera, the gargantuan flying turtle with the fiery bad breath. The answer seems to be: There have continually been fifth-graders spending their allowance on the Saturday creature double feature.

Long before teenage mutant ninja turtles turned up in New York City’s sewers, Gamera, the granddaddy of the species, was disturbed from his arctic slumber by an atomic bomb in 1965’s “Daikaiju Gamera.” Daiei Films’s unlikely turtle was a blatant knockoff of rival Toho’s successful “Godzilla,” released a decade earlier.

The movie cribbed everything, from the stomping of cardboard cities to Gamera’s grating roar. The anti-nuclear moral, however, wasn’t as clear-cut: The Japanese willingly borrow American atomic missiles to stop the terrible terrapin. A subsequent overseas version called “Gammera the Invincible” added an extra ‘m’ as well as scene chewing American actors.

Viewed as a kitschier alternative to the “Godzilla” series, “Gamera” was a big enough hit to spawn sequels every year until 1971, when Daiei’s productions suddenly went from low budget to no budget. In these films, Gamera battles an increasingly cheesy array of behemoth baddies, which later made him a favorite of “Mystery Science Theater 3000.” The lumbering turtle’s ability to fly, discus-like, through the air using his jet-propulsion shell, combined with “his unusual and overpowering kindness to children” can be downright groan-inducing. The low point was “Gamera vs. Guiron,” when the big green guy vanquished the enemy with a goofy gymnastics routine on a horizontal factory pipe.

When Daiei revived the franchise in the mid-1990s, they essentially started from scratch. Director Shusuke Kaneko, who had grown up on the kaiju classics, complemented the old techniques with innovative CGI effects. Visceral action sequences featured crayola-colored blood spurting from Gamera’s wounds; real tanks (not motorized models) caused explosions larger than cheap firecrackers.

By the time we get to “Gamera 3: Revenge of Iris,” the conventions have been reworked to include anime-inspired human telepathy. There’s still plenty of monster mayhem in the streets of Tokyo, but it now comes with the unflinching depiction of innocent people being visibly flattened and flung through the air. Finally, the Cold War atomic metaphor has been replaced by 20th-century sermonizing about mankind’s destruction of the environment.

Despite its inauspicious beginnings, the re-invented “Gamera” series is vastly superior to any recent “Godzilla” picture, including Roland Emmerich’s 1998 big-budget remake. Apparently, for this giant tortoise, slow and steady wins the race.

“Gamera 3: Revenge of Iris” plays at the Japan Society tonight at 6:30 p.m. (333 E. 47th Street, 212-832-1155).