The manga follows Ramba Ral, the charismatic Zeon soldier known as the Blue Giant from the original Mobile Suit Gundam, in a humorous spin-off set within the Universal Century timeline. After every shift, the formidable warrior secretly works as a nighttime cleaner at the Eden Club, a bar or entertainment venue that has closed for the night. Once his cleaning duties are complete, he indulges in his true passion: cooking and savoring decadent, unhealthy meals in secret, away from the watchful eye of his wife.

The core premise focuses on Ramba Ral as a lonely gourmand who transforms the closed club into his personal sanctuary for culinary exploration. Each chapter features him crafting a new, indulgent dish using readily available ingredients, such as bacon fried rice with Camembert cheese, Gundam-shaped hamburgers, or gyudon sandwiches with pickled ginger and tartar sauce. The narrative arc is episodic, driven by his joy in creating and consuming these hearty, often guilt-ridden meals. The series derives its title, which translates to Ral Meshi: Ramba Ral's Immoral Meal, from this central conflict between his battlefield heroism and his hidden, almost forbidden pleasure in rich food, a secret hobby that has visibly added to his sturdy physique.