Teenage Mutant Ninja Turtles (2003) - Season 1 Episode 10 -

Shredder attempts to convince Leonardo that the Foot are fighting the good fight and the turtles should stand with him. Leonardo is taken in and almost convinces the others until Splinter reveals what he knows of the Shredder. Leonardo realizes he's been fooled - it's never a good idea to fool a turtle at least not a Teenage Mutant Ninja one.