After the success of the first OVA in promoting the Supermutant toy line, the Mutant Turtle: Choujin Densetsu-hen series added a second episode. In 1997, the OVA The Coming of the Guardian Beasts – The Metal Turtles Appear! was produced to market the next major TMNT toy set in Japan – the Metal Mutation Turtles.
The episode begins with the Turtles, Master Splinter, and April O’Neil arriving in Japan for vacation. Meanwhile, Krang locates a new Muta-Stone, and sends Shredder, Bebop, and Rocksteady to find it. After immediately bumping into the Turtles on a bullet train, the goons are fended off by a group of ninja who once trained under none other than Master Splinter! The ninja leader, Kinzo Hattori, summoned his mutant friends to Japan to protect a mirror inlaid with seven Muta-Stones.
           The next morning, Shredder arrives at Hattori’s temple to take the mirror. In the ensuing scuffle, Hattori blows up the small temple to reveal a larger hidden temple with the mirror safely at the top. Good and evil scale the building (in total defiance of gravity and logic), with Shredder’s group arriving first. Hattori Kinzo is ready, though, and scatters the real mirror amidst some fakes. Unfortunately, Shredder finds the mirror first, and three Muta-Stones activate. Shredder, Bebop, and Rocksteady are given dark Metal Beast armor of a white tiger, a fish, and a hydra, respectively.
Of course, the Turtles retrieve the mirror shortly thereafter, and are granted armor of their own: Leonardo the Blue Dragon; Raphael the Red Phoenix; Donatello the Purple Manticore; and Michelangelo the Orange Spider-Tortoise. The Metal Mutants battle it out for the mirror and Muta-Stones, with both sides emerging victorious.  Shredder escapes with three of the Muta-Stones, leaving it up to the Teenage Mutant Ninja Turtles to use their new armor to retrieve them and save the world.
As a marketing mechanism, this episode was even better than the Supermutant OVA. It not only adequately displayed a brand new toy line, but it left the story’s end open enough to allow for the battle to continue in the basements of children everywhere. However, it was also a major problem for the TMNT Universe. While the first OVA was silly, it didn’t do anything that disrupted continuity or disturbed established plot points.
The second did both of those things. It had all the markings of a great addition to Teenage Mutant Ninja Turtles, but decided to take a lot of liberties. For instance, no mention of Kinzo Hattori training with Splinter and the Turtles occurred prior, and arriving in Japan on a plane in broad daylight flies in the face of Ninja Rule #1. Additionally, there were several points established that were never explained in any way (although, some probably couldn’t be worried about in the 28 minutes allotted for the episode). The armors drew heavily on mythical creatures, including the four guardians of Chinese and Japanese legend (the dragon, phoenix, white tiger, and tortoise). So why weren’t those four given to the Turtles, with the other three landing in Shredder’s hands?
OVA episodes are supposed to be fun and throw caution to the wind with their material. This episode showcased just how popular the Ninja Turtles had become, and did its job of pushing new toys well.  However, its attempt to become an established part of Turtles lore was a flop. With more episodes following this Metal Mutation storyline, it could have made for an excellent miniseries that would have worked well with the TMNT show as it had been developed over the previous decade in America. However, as a standalone episode, it left a lot to be desired. It’s probably for the best that this episode never hit the U.S., no matter how much fun the Metal Mutants would have been to play with.