The plot of Turtles Forever reminds me of the recent DC animated movie Crisis on Two Earths. The turtles discover that there is a multiverse of worlds where the turtles are present in a myriad of variations. The Shredder of the Turtles 2003 universe tries to reach the primary world, because he believes if he destroys that, the turtles will be gone forever, all over the multiverse. Either he doesn’t care or can’t believe that his actions will also destroy the rest of the multiverse with it.
That story is actually one of the smartest moves to cross the continuity of the older animated turtles series with the more recent series from 2003. Part of the charm of the movie is seeing the two sets of turtles interact with each other. If you have grown up with the old series you’ll feel like revisiting old friends. But the crossover of old and new turtles isn’t all there is to Turtle Forever.
The discovery of the multiverse allows the a visit to the primary turtle world. And here lies the biggest strength of the movie (compared to the DC movie Crisis of Two Worlds). There’s no bad pop-science involved, this is all about metafiction and crossing the different adaptations of the turtles. The primary turtles world is in black and white tones and represents the world of the first published turtles independent comics.
Especially clever is how the 2003 turtles are portrayed in comparison to the originals. The movie plays with the stereotype that the 2003 version is much more violent and rough, and shows by comparing them to their originals that they are still pretty much mainstream, just as much as the set from the first animated series.
That said, the movie doesn’t try to say this or that version of the turtles is better. It’s mostly having some clever fun with continuity and tries to show how far the turtles have come from their humble origins. This seems like a movie from fans for fans that also manages to be fun and entertaining for those who aren’t into turtles.