Struggling prog-rock musician William Brown finds himself in a living nightmare when he accidentally kills Vlad, the neighbor from hell.
William Brown, a neurotic, self-absorbed musician determined to finish his prog-rock magnum opus, faces a creative roadblock in the form of a noisy and grotesque neighbor named Vlad. Finally working up the nerve to demand that Vlad keep it down, William inadvertently decapitates him. But, while attempting to cover up one murder, William's accidental reign of terror causes victims to pile up and become undead corpses who torment and create more bloody detours on his road to prog-rock Valhalla.
Together with his girlfriend Emily, William Brown, a sound engineer and budding musician, resides in a shady apartment complex. William has spent the last three years working tirelessly on a frog-rock album in his spare time, but he has had trouble finding a market for it. One day, William discovers that Vlad, an Eastern European man with mental instability, has moved in with him after he gets home from the recording studio where he works. With his constant yelling, beating on the walls, and playing loud EDM music even at night, Vlad soon establishes himself as a serious nuisance.
After some time, William becomes tired of the commotion and visits Vlad in his apartment to confront him. But when he asks him to stop playing his music, Vlad sneers at William and teasingly challenges him to a fight, which ultimately results in Vlad unintentionally beheading himself with his mobile gym. William determines to dismember Vlad's body and leave it in his bathtub because he is disturbed and unsure of what to do. But after he does, Vlad mysteriously resurrects and informs William that he now "lives in his head rent-free".
Following his unsuccessful attempt to dispose of Vlad's remains at a smelting plant, William's boss summons him to work. Auggie, a vagrant who lives outside the recording studio, is run over by William's van when Vlad gets there. As Auggie passes away, William discovers to his dismay that Auggie is actually one of his favorite musicians, "Swig" Anderson. However, Auggie also mysteriously comes back to life a few moments later, just like Vlad.
William makes the decision to abandon Vlad and Auggie's remains in the middle of the forest, by the side of the road. Once he gets back to his apartment, William discovers that Eleanor, the building manager, killed herself by electrocuting herself while attempting to fix a malfunctioning fuse box. Because he has more electrical work experience, Eleanor had asked William to help fix the fuse box earlier, but he declined because he was preoccupied with dealing with Vlad's mutilated body. This makes William feel bad. Tensions start to develop between Emily and William in the meantime, as the latter becomes angry at the former for prioritizing his album over their relationship. William discovers that Vlad and Auggie have returned after Emily leaves, and they become friends after having an intoxicated party at Vlad's apartment.
William chooses to perform a song for Auggie and Vlad. William is delighted to see how much the song impresses both men. Then, though, William's neighbor Phillip shows up and yells at Will to stop playing music. After a fight breaks out, William kills Philip in self-defense. William is later taken to the mortuary to identify Eleanor's body. William, who feels he is to blame for Eleanor's death, sincerely apologizes to her after she returns from the dead. Eleanor, however, assures William that it is not his fault and offers to assist him in finishing his album with the assistance of the other zombies after the two of them sneak back to the apartment building. William is ecstatic and takes the zombies to the recording studio, where they successfully finish his album in spite of a few small setbacks.
