Complex sentence: Gold wrote a single episode of the cartoon show "Hey Arnold", in which title character Arnold stages an amateur magic show and "disappears" his friend Helga, who escapes during the trick, causing Arnold and the others to think she really has disappeared.
Split into simpler sentences: Gold wrote a single episode of the cartoon show "Hey Arnold". In it, the titular Arnold staging an amateur magic show and "disappears" his friend Helga, who escapes during the trick, causing Arnold and the others to think she really has disappeared.