Teenage Mutant Ninja Turtles (2003 Series)-Season 3-Episode 73-"Same as it Never Was"
In this episode the Ultimate Drako (Ultimate Ninja and Drako fused from the events in the Big Brawl part 4 Season 2) invades the Ninja Turtles liar and sends them all to different dimensions. This episode focuses on Donatello's separation in which he is 30 years in the future where he split, the TMNT are no longer together, Splinter is dead, Casey Jones is dead, Mikey has one arm, Hun and Stockman are conjoined, Leonardo and Raphael are at each others throats, and did I mention that Shredder rules the world? Well Donnie's return gets a now more series Michelangelo's attention and he explains to Donnie what has been going on since his departure. They find a way to get Leo and Raph together and they go into their hideout where April is in charge and battle hardened. Hun and Stockman now conjoined and in a wheelchair are now allies to the survivors. They make a plan to confront the Shredder and end him once and for all. In this battle Donnie watches all of his brothers die at the hands of Karai and her Karai bots. Hun cowardly tries allying back with the Shredder just to meet his own doom bringing Stockman with him. Donnie still young and in a mech suit (built using his genius) battles Shredder who has also had a makeover and finally destroys Shredder (at least in that existence). Donnie shares some words with April and finally fades away into his own dimension.
That episode had to be one of the series' darkest.