#unstable mutation

As the name suggests, the basic plotline of the arc involves the turtles' mutations becoming unstable, causing them to slowly transform into much more monstrous mutants, giving them a boost in abilities and physical prowess, but at a cost of intelligence and morality. In short, they get big and feral.

What I love about this arc is that it is actually paced rather well. They hint at it later on in season 8, follows through a slowly progressing destabilization throughout season 9, and then finally concludes in season 10 (the final season of the series).

It starts with them transforming suddenly, and very briefly at first, affording them a boost in strength and endurance, before they change back to their usual selves. However, another side-effect of the transformation is that it tires them out. Their first transformation literally knocks them out afterwards. As the story goes on, the transformations stay longer and of course often happen at very unfortunate moments, and it begins to effect their mental stability. As it progresses throughout the season, the unstable mutation threatens to become permanent as they lose themselves to the transformations.

Spreading the arc out in this way manages to keep up the tension really well!

Another cool aspect about the transformations themselves is that it's the first time that the turtles really look different from each other. For instance Leonardo gets some metallic-looking bits on his plastron and Donnie gets a cyborg eye. And I honestly wonder if "Rise of the TMNT" took some prompts from Raphael's unstable mutation design, because he's much spikier than the others.

The whole arc comes to a head when Donatello manages to find a way to stabilize their mutation, but due to some villainous shenanigans, Leonardo's transformation gets WORSE and they have to deal with that for a couple episodes.

The consequences of the arc even ties into the villain's main plot in the end, so it actually fits together quite nicely as a whole. The character Carter, that I mentioned earlier, also has a major plot to play in the whole arc, too, and he deals with similar mutation issues throughout. There's just so much going on in these seasons and it's epic and angsty and great!

If I had to choose, I'd still say season 1 of TMNT 1987 is my fave overall, but Season 9 (where most of this arc takes place) is a very close second, and it's largely because I think the whole "unstable mutation" idea was really well-executed!