If you look real carefully in the D'Hoonib level, after you blow up an aircraft, there's a poster behind advertising FOX Box's Teenage Mutant Ninja Turtles (2003) animated TV series on Saturday mornings.
This games is based on primarily Season 2 of Teenage Mutant Ninja Turtles (2003) the animated series. Although the first few stages are based on the late Season 1 episodes Return to New York: Part 3 (2003), Search for Splinter: Part 1 (2003) and Search for Splinter: Part 2 (2003), the rest of the game is based on Season 2 with stages in the game based on following episodes: Turtles in Space: Part 1 - The Fugitoid (2003), Turtles in Space: Part 2 - The Trouble with Triceratons (2003), Turtles in Space: Part 3 - The Big House (2003), Turtles in Space: Part 4 - The Arena (2003), Secret Origins: Part 1 (2004), Secret Origins: Part 2 (2004), Secret Origins: Part 3 (2004), Return to the Underground (2004), The Ultimate Ninja (2004), What a Croc (2004), City at War: Part 3 (2004), Rogue in the House: Part 1 (2004), Rogue in the House: Part 2 (2004) and some other levels made up for just this game.
After completing a task that can unlock character like Karai, Casey Jones, Splinter and Slashuur, you have to wait 5 - 45 days after before those characters become available. If you change your video game's time setting, you can speed up the process so don't have to wait a month and a half to play the character that takes the longest wait to unlock.