At Flo's V8 Cafe, the cars are enjoying karaoke night (Guido is currently singing, with the karaoke displaying Lightning & Sally) and Lightning McQueen tells Mater to sing. Mater replies "Nah, I don't want to steal the show," and then he starts his story. Maters tells Lightning that he used to be a rock star. He started out in a garage band called Mater and the Gas-Caps with Mater as the lead singer, Rocky & Eddie at the electric guitar and bass guitar, and a drummer Pitty. They first performed at the Top Down Truck Stop (a location that first appeared in a deleted scene from Cars and also made a cameo in the film), and when they finished, Mia and Tia asked them if they had a record. Realizing they did not have a record made yet, the band heads over to a recording studio to make one. Everything went well in the studio until a fly got in. When the drummer tried to swat the fly with his drumsticks, he started playing faster. Mater had no choice, but to sing faster too, he liked the new heavier sound. A music agent named Dex liked their new sound too. Dex said all they needed was a new name. Mater thought for a while, but before he could answer a package arrived. "Where do you want this Heavy Metal, Mater?" asked the mover. "That's it!" cried Mater. And Heavy Metal Mater was born. The band's concerts sold out wherever they went. The band all had a cool new look that drove the fans wild. Heavy Metal Mater always put on an amazing show. They had lights, guitar solos, and they closed every show with their most popular song, "Dad Gum". And just as they finished the song, a giant Heavy Metal Mater balloon would float over the audience. "You were Heavy Metal Mater?" asked Lightning. "No," replied Mater. "We was Heavy Metal Mater!". Lightning was in the band too, jamming on the keyboard. He even joined Mater at the microphone for the big finale. "I'm sorry Mater, that did not happen," said Lightning. "Well suit yourself," replied Mater as he looked up in the sky where there was the giant Heavy Metal Mater balloon.
- Lightning McQueen
- Sally Carrera (karaoke scene only)
- Drummer Pitty
- Rodney the Rocker
- Mia and Tia
- Vroomaroundus Bugus
- When Guido is singing karaoke at Flo's V8 Cafe, seen at the left of Guido is a rod with red, yellow and green lights that was originally from Unidentified Flying Mater.
- To the left of the garage used by "Mater and the Gascaps," the pool from Mater the Greater can be seen.
- The Top Down Truckstop, where "Mater and the Gascaps" played, was originally seen in the movie Cars when Mack starts to get tired and wants to rest.
- The statue of Stanley makes a cameo in Heavy Metal Mater at the end of a corridor when Dex hears the new sound of "Mater and the Gascaps."
- The final concert of the short takes place at the Motor Speedway of the South, the stadium where the first race of the movie Cars takes place. This is the third time it made an appearance in a Cars Toon episode, the first time being Mater the Greater and the second one Monster Truck Mater.
- Dex is supposed to represent Tex Dinoco.
- This is the only Cars Toon released at the time and not new to the Cars Toon DVD/Blu-ray release that is not represented in the Cars Toons: Mater's Tall Tales video game.