The fourth season of Teenage Mutant Ninja Turtles 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 this season focused on Leonardo, the group's leader, who became bitter, 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. This season is known among of fans as the darkest season of the series.