Mustard joins the League of Villain's Vanguard Action Squad following the Hero Killer's defeat. While the team waits in a warehouse to be dispatched on their first mission, Muscular mentions that they need to hurry because he is ready to fight. Unmasked, Mustard replies that they have been ordered to wait and must refrain from doing anything to stand out.
Most of the Vanguard Action Squad arrives at the Wild, Wild Pussycats' Beast's Forest reserve and observes the U.A. High School students training there. Himiko Toga complains about the style of her new gear and Mustard tells her to worry about its functionality, not it’s fashion. The next night, the mission to capture Katsuki Bakugo commences. Mustard fills a large area within the forest with the poison gas produced by his Quirk. Juzo Honenuki is one of many students knocked out by the gas.
Mustard feels the presence of two people approaching him as they rush through the typhoon of gas he has created. Impressed the students have located him, Mustard reaches into the folds of his uniform and says he expects no less from U.A. students. Otherwise unconcerned, Mustard pulls out a handgun and says that no matter their Quirks, the students are only human. Tetsutetsu Tetsutetsu suddenly comes charging through the gas and Mustard counters by swiftly shooting Tetsutetsu's gas mask off his face.
Mustard recognizes him from the U.A. Sports Festival and remembers his Quirk transforms his skin into steel, making him bulletproof. Even so, Tetsutetsu can only go for as long as he can hold his breath while inside the field of toxic gas. Tetsutetsu tries to rush an attack, but Mustard punishes him by shooting him and telling him he should be smarter as a U.A. student. Mustard aims his gun at Itsuka Kendo, who ventured with Tetsutetsu. He shoots, but Tetsutetsu takes the force of the bullet for her. Mustard mocks them for their simple surprise attack when they have him outnumbered. He explains that he can track their movements through the fluctuations in the gas and conceals himself within it.
Mustard continues to mock U.A. High while hiding within the toxic miasma. Tetsutetsu attempts to follow him, only for the masked villain to appear by his side and shoot him in the head again. Mustard notes that the student's metal skin seems to be weakening, and believes it may be because of fatigue and the lack of oxygen. He labels Tetsutetsu as a simple one track mind with a one trick hardening type ability that acts too recklessly. Mustard finds it strange that people praise students like him just because of his school. He shoots Tetsutetsu repeatedly and kicks him over as he declares his previous statement is what is wrong with the world.
Itsuka tries to attack Mustard, only for him to easily evade and remind her that he can track her movements. Itsuka enlarges her fist to great size and manages to knock Mustard back, and he backs into the safety of the gas and mocks her pitiful Quirk. He is surprised when Itsuka uses her enlarged hands to create a whirlwind by swiping them back and forth. Itsuka taunts Mustard about using a gas mask and a gun in a fight, saying he has no confidence in his own fighting ability.
Angered, Mustard aims for her until he notices too late that Tetsutetsu is rushing him from behind. He is unable to react in time because Itsuka thinned the gas around them. Tetsutetsu uses the opening to land a powerful, steel enhanced punch on Mustard. He crushes Mustard's mask and knocks him unconscious. Mustard's gas begins to disperse immediately after his defeat.
Some time afterward, Mr. Compress' message concerning the success of their mission is broadcast to all of the members of the Vanguard Action Squad, with Mustard having been moved into a sitting position. Later, after the rest of the Vanguard Action Squad escapes, the defeated members of their group are left behind, and Mustard is dragged through the forest by Itsuka. He is then arrested by the Police Force alongside Muscular and Moonfish.