You (Donatello) along with Raphael, Mike, and Leonardo have always been mutant turtle ninjas, trained by Master Splinter.

After drinking some juice in the sewer, you all transform into humans for the first time.

Splinter thinks it might be a curse and tells you to find a cure in a laboratory outside the sewer before it's too late.

Leonardo clenches his fists and says: "Alright team, this isn't the endgame! Let's find that lab and turn back before we miss kickoff!"