Day of the Rat is a story written by Generic Writer's Name, and published on Fanfiction Net between 4 September and 24 December 2016, first titled Teenage Mutant Ninja Turtles IIIish: Day of the Rat (1992) The Novelization. It was also published at Deviantart in mid-2017, as a downloadable pdf novel. It is set in the 1990's TMNT movie universe between Teenage Mutant Ninja Turtles II: The Secret of the Ooze and Teenage Mutant Ninja Turtles III.
Shortly after the events of The Secret of the Ooze, even as the depleted Foot Clan are starting to rebuild their shattered empire, a new threat is emerging from the vast sewer system beneath the streets of New York. The Turtles are mysteriously absent from the city - possibly to get away from their newfound fame, or possibly on some last great test before they become true ninja...
The Turtles are needed more than ever. By the time they do return, the situation has grown even more critical - Waves of invading rats are on the loose, directed by a single intelligence. April O'Neil, investigating the return of the Foot, has been captured by them. Master Splinter is falling under an external control. As New York is conquered around them, could the Turtles really have to face their own sensei in combat..?