Brütal Legend follows the story of Eddie Riggs, who is living as a roadie for his favorite Heavy Metal band, Brütal Legend. The game begins one night before a gig. As Riggs is tuning a guitar, an accident happens and he starts to bleed, his blood dripping onto his cursed belt buckle, which bears a strong resemblance to Motorhead's Snaggletooth emblem. Before he knows it the beast on his buckle has come alive and transported him into a Heavy Metal and Nordic fantasy inspired alternate history where demons have enslaved humanity.

After escaping the Temple of Ormagoden where he first appears in the Heavy Metal past, Eddie meets Ophelia, a love interest likened to the same character from Shakespeare's Hamlet. Ophelia is soon severely injured and Riggs must climb a mystical mountain to do battle with a motorcycle driving bass guitarist called the Kill Master, whose riffs are said to heal wounds.

Brütal Legend is a third person action game, the gameplay is similar to others in the genre. Eddie Riggs wields a battle axe as a primary melee weapon, while his second weapon is a guitar that he brought back with him from the real world. A forward attack sends Eddie attacking his enemies with the battle axe, while a wider slash attack takes out a larger quantity of enemies. Using the guitar, Eddie Riggs can play it to defeat his enemies. The guitar is used as a ranged assault weapon, when wielded players can perform a guitar solo in response to your combo-styled button presses. Attacks are based upon the combo pressed in, possible combos include a pyrotechnics display where flames come from the ground to burn your enemies. A third weapon is the Deuce, a drop-top coupe Eddie builds out of spare parts he finds. The Deuce can be used to run enemies over, but it is also used to traverse the large game world. Ancient garages can be used to upgrade the Deuce with weapons and abilities. A large part of the gameplay involves controlling an army of metal-themed characters, such as thick-necked Headbangers and large-fisted Bouncers. When Eddie defeats a type of character, they will then join his army. Each type has it's own attributes. Simple commands such as "Defend" or "Attack" are issued to the army by use of the D-Pad.