Zombietale is an AU created by zombie-frisk. The only change is that Frisk is now a zombie. It is not known how they become a zombie, but theories include that the earth is now post-apocalyptic where zombies rule supreme or that Frisk had died in the fall and returned as a zombie.
They are soon found by Toriel and is taken to her home. She spends time with Frisk, reads them stories and bakes them pies. However, Frisk is filled with hunger.
One night, they leave the house and feed on a Froggit, transforming it into a zombie. The Zombie Froggit goes on a rampage, biting other monsters and spreading the infection. A Whismur named Sunny escaped the destruction with the help of Flowey and she is taken under his wing.
Toriel eventually finds the infected monsters in the ruins. She runs away and locks herself and Frisk in her home. She warns Frisk about the monsters. When she realizes the truth that Frisk was actually the cause of the infection, she fights Frisk in order to protect the monsters.
Frisk spares her and Toriel took them into a hug, promising that they would fix everything. But Frisk is filled with hunger and bites the monster when she least expected it, leaving them feeling hungry and empty.
Frisk is the main character of the AU and is the only one that is changed from the original Undertale. As they are a zombie, they cannot die. Healing magic does not work on them and only causes them pain. While they seem to only care about their hunger, they have been shown to feel regret about their actions.
Flowey is relatively unchanged from Canon, but he plays another role in Zombietale. He is somewhat of a hero, saving Sunny from the zombie hoard and figuring out ways to actually kill the zombies.
That was, before his untimely zombiefication after coming in close contact with Frisk, who he mistook as Chara.
Sunny is a Whismer that managed to survive the zombie apocalypse. Her sister, Luna, was killed by the Zombie Froggit. She was saved by Flowey, who stumbled onto the scene and carried her under the underground where it was safe.