Tomura Shigaraki receives the location of U.A.'s Quirk Training Camp from All For One. Giran meets with Tomura and Kurogiri to confirm a shipment of equipment for the League of Villain's new Vanguard Action Squad. With all of his pieces and prey in place, Tomura sets his evil plan into motion.
The supplies are delivered to Tomura's new allies. Dabi, Himiko, Muscular, and Mustard meet an undisclosed warehouse where they receive their new gear. Muscular wants to act but Mustard reminds him that it's too early to attack and that the squad isn't supposed to do anything flashy.
Dabi mentions that Tomura acted like a spoiled brat when they met, but now he's taking charge. Eager to follow Tomura's lead, Dabi says that the coming attack is a warning shot to condemn the false heroes and secure a new future for society.
Several days later, the Vanguard Action Squad rendezvous on a mountain overlooking the U.A. Students camp in the woods. Equipped with new gear engineered from the black market, seven of the villains get together and prepare the invasion. Once again, Muscular states he's eager to attack, but Dabi says they will wait for the three remaining villains to join their party.
Dabi states that the League of Villains made a mistake during the U.S.J. Incident by relying on a bunch of random thugs. He believes a small group of elite villains is more effective at destroying their enemies sense of peace. 
The Vanguard Action Squad invades the Beast's Forest on the third day of U.A.'s Quirk Training Camp. They start the attack when Mustard fills the forest with poisonous gas and Dabi sets another large section of it ablaze. This signals the others to start moving out. Spinner and Magne are tasked with dealing with the Wild, Wild Pussycats hero team. Magne quickly subdues Pixie-Bob by smashing her head with a large iron rod and knocking her out. Muscular searches for a vantage point and happens upon Kota Izumi overlooking the burning forest. 
Mandalay uses her Telepath Quirk to broadcast a warning to everyone about the villain attack. She tells the students not to engage and to make their way back to camp. Twice creates a clone of Dabi to attack the Base Camp in order to keep Eraser Head and Vlad King busy. When Shota receives Mandalay's message he rushes outside, where he's attacked by Dabi. Mandalay and Tiger fight Spinner and Magne after telling the students to return to base camp. Izuku asks Mandalay's permission to find Kota and she agrees.
Inside the poison gas filled forest, Momo creates gas masks with her Quirk and tries to reach her fellow students. A few students fall victim to the gas and are knocked unconscious. Tetsutetsu and Itsuka are among those who survive the gas and the former decides they need to stop the gas user. Izuku reaches Kota in time to save him from Muscular.
Izuku went off alone to save Kota and he breaks his phone saving him, so he has no hope for backup. Even so, Izuku decides to face Muscular alone in order to protect Kota. Muscular is the same man who murdered Kota's parents and skewed the young boys view on hero society. Shoto finds Kosei Tsuburaba unconscious in the forest and carries him on his back. He and Katsuki happen upon Moonfish after the villain attacks Fumikage and Mezo 's group.
During the fight between the Pussycats and the villains, Mandalay notices that Ragdoll isn't responding to outgoing messages. Magne hints at the fact that Ragdoll was attacked by Nomu. Muscular dominates Izuku at the beginning of their fight and toys with him. The villain also mentions that the villains are looking for Katsuki. When Izuku learns of Muscular and Kota's connection, he becomes enraged and attacks Muscular with One For All at 100%. 
Muscular survives the attack, much to Izuku's surprise. The villain decides to take the fight seriously and attacks Izuku with his full power. He nearly crushes Izuku, but Kota distracts him with his Water Quirk and pleads for Izuku's life. Izuku uses the opportunity and defeats Muscular with One For All 1,000,000% Delaware Detroit Smash. Izuku's selfless act teaches Kota what it means to have a hero. 
Eraser Head restrains Dabi and tries to interrogate him, only to discover he is a fake. He meets Izuku in the forest, takes Kota from him and tells him to return to base camp. Izuku refuses because he has to try and save his classmates. Knowing the students are the League of Villain's target, Shota tells Izuku to find Mandalay and tell her that the students have permission to fight the villains. Izuku finds Mandalay and relays Shota's message. She uses Telepath to tell all the students they have permission to defend themselves. 
Magne tries to kill Izuku but Spinner stops him because Stain had deemed him a true hero. They're both struck down by the Pussycats while they're distracted. Katsuki tries to fight Moonfish but he's unable to get close. Shoto defends them both with his ice, but he can't land an attack because Moonfish is too experienced in battle.
Itsuka and Tetsutetsu search the forest for the gas user while Izuku looks for Katsuki. Izuku is suddenly attacked by a rampaging Dark Shadow. Mezo comes to his rescue and reveals that Fumikage has lost control over his Quirk. Mezo explains that Fumikage lost control after Moonfish attacked them both and severed one of Mezo's Dupli-Arms. Fumikage tells them both to stay away but they refuse to leave their friend behind.
Tetsutetsu finds Mustard at the center of his gas typhoon. Mustard shoots his mask off and evades the student's attacks by reading the flow of the gas. They fight until Itsuka dissipates the gas by flailing her Big Fists Quirk. Using the opening, Tetsutetsu manages to defeat Mustard with one powerful punch. Mustard's defeat clears the forest of the gas completely.
Shoto struggles to defend himself from Moonfish's onslaught of teeth attacks. He and Katsuki are both surprised when Dark Shadow is led into the area by Mezo and Izuku. They decided to lead Dark Shadow to Katsuki so they could save both of their classmates at once. Dark Shadow completely annihilates Moonfish and then Shoto and Katsuki use their Quirks to produce enough light to tame Dark Shadow. Izuku and the others form the Bakugo Escort Squad to safeguard Katsuki since they know he's the target of the Vanguard Action Squad.
Himiko Toga attacks Ochaco and Tsuyu and tries to collect their blood. She's subdued by Ochaco's Gunhead Martial Arts but manages to distract the student long enough to drain some of her blood. Izuku and the others arrive and force Himiko to retreat. Then, Mr. Compress reveals himself and he manages to steal both Katsuki and Fumikage with his Compress Quirk. 
Mr. Compress radios his allies that he's captured Katsuki and they all move to the rendezvous point. Nomu nearly kills Yosetsu and Momo but Dabi calls it back just in time. The students use their Quirks to plant a tracking device on Nomu as it retreats. Mandalay and Tiger manage to defeat and restrain Spinner and Magne, but Kurogiri arrives to retrieve them. Dabi and Twice meet Himiko at the rendezvous point. They are all present as Izuku, Shoto, and Mezo tackle Mr. Compress to the ground. 
Another one of Dabi's clones attacks the base camp and confronts Vlad King. Vlad King restrains it with his Quirk and Dabi taunts him about the impending destruction of hero society. Shota arrives to drop off Kota and destroys the clone himself. Eijiro and Tenya ask for permission to fight but they're both denied.
The Vanguard Action Squad engages the Bakugo Escort Squad in battle. Mezo manages to take the two marbles he believes Katsuki and Fumikage have been trapped in. Nomu and Kurogiri arrive on the scene and stop the boys from escaping.
Mr. Compress reveals he was hiding the real marbles in his mouth. Kurogiri warps most of the villains away and Yuga stops Mr. Compress by blasting him with a Navel Laser. The villain drops the marbles and Mezo manages to retrieve one. Dabi grabs the other just before Shoto can get to it. Mr. Compress releases his Quirk and Dabi drags Katsuki through the warp-gate, completing their mission. 
Fifteen minutes after the villain's retreat, the Police and emergency services arrive. Of the 40 students who were attacked, 15 were knocked unconscious, another 11 were injured, one was captured and only 13 of them remained unscathed. As for the Pro Heroes, Pixie-Bob was in critical condition and Ragdoll was missing. Muscular, Mustard, and Moonfish are all arrested by the Police.
The injured students are sent to the hospital. U.A. High is disgraced by the media for allowing another villain attack on their students. Nezu holds a meeting with his faculty to discuss their desperate situation. He declares that the villains have officially started their war on Hero Society and they've dealt a serious blow to U.A.'s reputation. Present Mic believes there is a traitor on the heroes side but the other teachers dissuade him.
Two days after the event, most of Class 1-A visits Izuku in the hospital. There, Shoto and Eijiro reveal they plan to rescue Katsuki from the villains. 
- Tiger & Mandalay vs. Magne & Spinner
- Shota Aizawa & Vlad King vs. Dabi's Clones
- Izuku Midoriya vs. Muscular
- Tetsutetsu Tetsutetsu & Itsuka Kendo vs. Mustard
- Katsuki Bakugo, Shoto Todoroki & Fumikage Tokoyami vs. Moonfish
- Ochaco Uraraka & Tsuyu Asui vs. Himiko Toga
- Bakugo Escort Squad vs. Vanguard Action Squad