Complex sentence: In the episode, Bart Simpson meets Chester J. Lampwick, who proves that he is the creator of Itchy from "The Itchy & Scratchy Show" and sues Itchy and Scratchy Studios, which he claims stole his idea.
Split into simpler sentences: In the episode, Bart meets Chester J. Lampwick, who claims and successfully proves that he is the creator of Itchy from "The Itchy & Scratchy Show". He sues Itchy and Scratchy Studios, the owner of the Itchy and Scratchy characters, which Lampwick claims stole his idea.