The episode begins with Mario beating up a Shy Guy and a Goomba at night. He hits a block containing a Poison Mushroom, which he wisely avoids touching. However, it soon touches the squashed Shy Guy and Goomba and turns them both into zombies. Mario runs off screaming while a nearby Koopa Troopa looks puzzled. He turns round and yells as the zombies eat him and turn him into a zombie too.
Hal is shown playing a videogame (Sonic) until Jeff comes in and tells them that zombies are eating people alive and spreading their undead virus. Hal blows this off until Jeff tells him they called Hal gay. The pair go to a Koopa salesman who owns guns and other artillery. Hal offers him 40 coins for one, but when the salesman says he doesn't have enough money, Hal picks up one of the guns and shoots him dead. Jeff is horrified by this, but Hal tells him that no one is going to miss a "creepy black market merchant" when there's a zombie invasion. Hal and Jeff soon come across a zombie Koopa Troopa and zombie Shy Guy (probaly the ones from the beginning), and begins shooting at them. Hal fails to shoot their critical point, the head, but Jeff soon vanquishes them by firing a cannonball into them. However, they are soon confronted by many more zombies and Jeff claims he is out of ammo for the cannon (and obviously can't fire a gun since he has no arms). When Hal tells him to use his head, Jeff unknowingly takes this literally by hopping in the cannon and firing his head into a horde of zombies, killing them all and creating a path for Hal. He tells Jeff that he meant "improvise" (suddenly replying "Jeff, That's not what I meant...shooting yourself into a horde of zombies?!") and not literally using his head, but Jeff reassures him by telling him it worked. A Dry Bones suddenly walks up behind them and Hal knocks his head off in panic. Dry Bones is annoyed by this and puts his head back on, and Jeff realises that he's actually the undead and not a zombie. Dry Bones tells them that there are a few more survivors in a castle on the Donut Plains and that Hal and Jeff should meet up with them. They ask if Dry Bones is coming with them, but he says he prefers hanging out with the zombies and that they're really friendly once they've eaten you. Seeing as Dry Bones is a skeleton and doesn't have any flesh, he has nothing to fear. Hal and Jeff take his advice and head off for the castle.
After reaching the castle, Wario lets them in and they meet Geno, the Inaudible Thwomp and Big Boo. Geno, Big boo and the Thwomp all make up stories as to why the zombies have appeared, but the others don't buy it. Suddenly, the castle is attacked by more zombies and Wario begins shooting at them. When he runs out of ammo, he charges at them headfirst, only to be bitten by a zombie Shy Guy. Big Boo disappears and the Thwomp rises into the air, leaving Hal, Jeff and Geno to fight alone. Geno is soon swallowed by the zombified Steve the Piranha Plant, but Hal and Jeff continue to battle with the remaining zombies until their ammo is completely gone. During the credits, loud cries and screams are heard, as well as munching sounds from the zombies. After the credits, a zombified Hal and zombified Jeff are shown eating Luigi whilst discussing the good and bad points of being a zombie. In the background, Mario is shown running away from the zombified Wario.
In the other ending, Geno destroys Steve and the other zombies with a powerful laser beam, saving Hal and Jeff in the nick of time. He then asks whether they missed him and that they now know why he's awesome.