Great episode! Go watch it, now. Probably one of the funniest episodes in a long while. I won't spoil too much of it here, but basically it is one story told in three different animation styles, 1940s black-and-white style, 80s 8-bit video game, and anime style... Stephen Hawking voices himself. And there are references to Voltron and Robotech in the final segment. They even use some of the music from Robotech! Oh, and Omaha, Nebraska is destroyed.
I loved the 8-bit video game segment the most (both the style and the story itself), but all three were incredibly well done.