During a visit of the War Minister's personal adviser to a secret base in the desert, Dr. Pretzel's secret 'ultimate weapon' invention, a bullet proof fabric, is stolen by Horace Twoface. Lucky Luke gets the job of retrieving it for the US government, which leads him to a tiny Mexican village near the Rio Grande, where the megalomaniac El General rules absolutely and pays well for such military advantages. Twoface also arrives there, in a car.