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After the whole 'Dropping a ton of mutagen canisters over New York' thing, there's been many cases of mutated animals and people that the turtles have had to face, and after not having any for a few months, they figured they've seen them all. But they're proved wrong when they hear of a report of something attacking sewer workers near their home, causing them to go and investigate.... let's just say, they find something very interesting.