Walt, envious of the attention his brother gets from the Turtles, steals a rare pouch containing the only physical map to the Turtles' secret lair.
The episode serves as a heavy-handed moral story about the dangers of theft and envy, with Walt eventually learning his lesson after putting both his brother and the Turtles in grave danger. Fun Facts & Trivia [S3E40] The Missing Map
If you were thinking of a different series, "The Missing Map" is also: Walt, envious of the attention his brother gets