While trying to contact the Turtle Nebula, Donatello meets an intelligent lizard alien named Algernon ("Algae" for short), who must help him and the other Turtles rescue April from a greedy man named Hostile Hiram Grelch.

Splinter has suddenly disappeared, and a note left behind from him forces the Turtles to go their separate ways, but one by one, they are captured by Leatherhead and the Rat King.