Teenage Mutant Ninja Turtles' fourth season originally aired between September 10, 2005 and April 15, 2006, beginning with the "Cousin Sid" episode. Fourteen random episodes from this season were released on DVD on September 12, 2006 Much of the series focused on Leonardo, the group's leader, who became more reserved and isolated following the final episode of the previous season in which he, his brothers and their master were almost killed. He went so far as to lash out at Splinter, causing him serious injury.