In 1993, with the Teenage Mutant Ninja Turtles brand already somewhat on the wane, Teenage Mutant Ninja Turtles III was released in theaters. Rather different than its predecessors, the movie sent the titular ninja turtles back in time for a relatively sleepy adventure in feudal Japan. With its lack of familiar villains and a noticeable drop in the quality of its special effects, TMNT III is considered by many fans to be one of the nadirs of the franchise.

Despite the failure of TMNT III to capture the public’s imagination, plans were put into motion to push ahead with a fourth installment of the series. Tentatively titled Teenage Mutant Ninja Turtles: The Next Mutation, the proposed film reached pre-production before descending into development hell and eventual cancellation. Over the years, plot and concept art leaked onto the Internet have indicated that the fourth TMNT would have been an extremely strange movie – a notion confirmed by a recent dump of character sketches by the turtles’ co-creator, Kevin Eastman.

Comic Book Movie recently took note of a host of Kevin Eastman-penned concept art up for sale at Heritage Auctions. Much of this concept art comes from work Eastman did on Teenage Mutant Ninja Turtles IV: The Next Mutation. Along with strange new takes on familiar characters, the set of images reveals an interesting hook for the proposed film: a fifth mutant turtle called Kirby, named for the King of Comics, Jack Kirby.

You can look over this concept art (minus a bafflingly not-safe-for-work “Evil April O’Neil”) in the gallery below. For the sake of context, we have included several sketches by the other creator of Teenage Mutant Ninja Turtles, Peter Laird. These sketches show the main four Turtles and their sensei, Splinter, in what would have been their “updated” forms. Laird’s art was posted on his personal blog in 2008.

Though not much is known about the full scope and plotline of TMNT IV, sparse details have emerged along with other pieces of concept art over the last decade. Supposedly, the fourth film was going to involve the Turtles and their allies undergoing “secondary” mutations that gave them new superpowers. These would have included Leonardo gaining the Colossus-like ability to coat himself in indestructible chrome; Donatello discovering telepathic and telekinetic powers; and Master Splinter growing into a hulking, combat-ready brawler. Some promotional materials for what was at the time an upcoming film also indicated that the movie might deal with parallel universes and science fiction technologies. And, of course, the film would have introduced another ninja turtle named for a famous artist, Kirby.

If one goes off of Eastman’s concept art alone, the nods to the legacy of Jack Kirby would have gone beyond the name of the surprise new ally (or enemy) of the sewer-dwelling heroes. The designs for both new characters and old (for instance, Casey Jones’ cybernetic fists) contain more than a taste of Kirby’s angular, often-uninhibited design aesthetic. There are also shades of the mid-90s fascination with Blade Runnerstyle cyberpunk in several of the sketches, as well as types of composition popularized by the grim-n’-gritty comic books of the era (i.e. – stiffly posed action; jagged talons; massive combat knives held aloft).

Looking over the collection of Kevin Eastman’s production sketches and other pieces of art afloat on the Internet, it becomes obvious why TMNT IV never went farther than the drawing board. While fun and highly atmospheric, the sketched characters reflect what would have been a major departure in terms of tone from the rest of the franchise. While the producers of the series may have wanted to shift the fourth movie into a grittier and more adult-oriented mode, changing Shredder into what appears to be a General in the KISS Army was probably not the way to go. In addition, the character of Kirby looks to have been only loosely planned at the point Eastman was sketching him – indicating that the “fifth turtle” probably only had a tentative role in that draft of the script.

After TMNT IV was scrapped, the title of The Next Mutation eventually passed to a new, live-action TMNT television series in 1997. That series made good on the promise of a fifth ninja turtle, introducing a female mutant named Venus de Milo. Most TMNT fans like to pretend this never happened. Whether the much-maligned lady turtle was any better as a fifth Ninja Turtle than the theoretical Kirby may end up being one of genre fans’ grand “What if?” scenarios.

Michael Bay’s Ninja Turtles is currently set to open in theaters May 16th, 2014. The Teenage Mutant Ninja Turtles television series airs Saturdays on Nickelodeon.

Sources: Heritage Auctions [via Comic Book Movie]