Marshall's darkest secret is that he really does need blood to survive, and Gumball has been enabling him for years, but there comes a time when enough is enough, and PG cannot keep enabling his friend. He's cut off, and is given the choice to speak to Fionna about it, or deal with the consequences on his own. Things do not turn out well. SCIENCE! And sex! And Vampirism! What's not to like? Probably the nasty business of drinking blood.