Musha Kero was a 17-minute short special in Keroro Gunso the Super Movie 3: Keroro vs. Keroro Great Sky Duel de arimasu!
The Keroro Platoon is sent to Sengoku Ran Star Planet by HQ to defeat the Warrior Viper because of, according to Giroro, Keroro's inactivity within the invasion. In this short movie, they don the Warrior costumes, which are later used in Keroro RPG: Kishi to Musha to Densetsu no Kaizoku. Dororo departs to check the nearby areas, and as he does so, Tamama points out a Dango shop, which he and Keroro immediately go to. The woman working at the shop appears to be Angol Mois, but she's actually the "village maiden type A." Keroro, however, doesn't know this.
They encounter fleeing villagers, who are followed by an army of the Warrior Viper's underlings. Once Tamama and Giroro defeat a multitude of the underlings, a smokescreen is put down and a machine resembling a snake emerges, the Warrior Viper residing at the top of its head. Keroro requests a Hellmageddon from the village maiden, but she gives him a "Round Bald Rice Bowl" instead and Keroro finally figures out that she is not actually Mois.
The Warrior Viper then decides that he wants to take the village maiden, but as he tries to take her with the machine, Keroro and Tamama are taken instead. When they are within the machine, Keroro and Tamama realize that the underlings of the Warrior Viper control its movement. Tamama immobilizes the machine by unleashing a noxious fart on the underlings and the machine appears to 'malfunction,' violently twisting and turning, and sends the Warrior Viper flying out of his place at the top of its head. The head and neck of the machine then come hurtling down, nearly landing on Giroro and the maiden, but their doom is prevented by the sudden comeback of Dororo, who cuts the neck to save the two.
With Kururu as his hostage, the Warrior Viper returns and demands that they give up the maiden, threatening to kill Kururu with the weapon on his arm. The Platoon appears to not care, saying that they'll have to sacrifice Kururu, and the Warrior Viper gapes in anger and confusion. The seemingly irritated Sgt. Major then takes the opportunity to throw the round bald rice bowl he was holding into the Viper's mouth, which he turned into extremely hot curry with all the curry supplies he brought with him. Once Kururu is freed from the Warrior Viper's grasp, the platoon enters into a short battle with the Viper. Dororo, Tamama, and Keroro defeat him with their "Jet Stream Attack," where they do consecutive attacks and slice away his armor and weapons. The Warrior Viper is left powerless and nearly naked, and flees with his army.
At sunset, the villagers gather in front of the platoon, and the maiden thanks them profusely. However, she says that they defeated the youngest Warrior Viper, and Dororo confirms that there are seven siblings for them to defeat. Keroro, Giroro, and Tamama appear to be devastated by this news. The short movie is then left off with a cliffhanger.