In Hell, imp Blitzø runs an assassin business targeting the world of the living, using a spell book borrowed from a prince of Hell. With employees Moxxie, Millie, and Loona, they attempt to survive each other while keeping business afloat.

Follow Blitz, a classic demon Imp who sets out to run his own small assassin business with his weapons specialist Moxxie, his bruiser Millie, and his receptionist hellhound Loona. Together they attempt to survive each other while running a startup in Hell.—Vivziepop

In Hell, Black is White, Right is Wrong and assassination is a Jack-of-all-trades job for the demons of the underworld. Join the Imps of Hell as they embark on journeys of death, sex and loyalty. Nothing is as it seems for these wonderful demons.—Johnny

The series follows the employees of I.M.P (Immediate Murder Professionals), an imp-run assassination company in Hell, on their many different jobs.The members of I.M.P include Blitzo (pronounced "Blitz"), the boss of the venture, along with weapons specialist Moxxie, powerhouse Millie, and receptionist hellhound Loona. With the help of an ancient book obtained from Stolas, a Goetial demon of Hell, they access the human world to complete their tasks on order from souls in Hell. The show features a different cast of characters and a different storyline than Hazbin Hotel, another show created by Medrano, despite taking place within the same universe. As Medrano described it, while both shows share the same setting, Helluva Boss follows "characters and societies that already exist in Hell", with the main focus being on interpersonal relationships between characters, while Hazbin Hotel is about redemption and consequences.