While schools in Japan are on vacation, Melissa Shield sends an email to U.A. Teacher and No.1 Hero: All Might. In her message, she asks All Might to come and visit her father on I-Island for an upcoming expo. She hopes that All Might will reunite with her father and she expresses that she can't wait to see his reaction.
Not long after All Might's arrival, Melissa is shown hopping on one of her advanced pogo sticks before spotting him in the park. Melissa reunites with him and tells him she grew up, Melissa invited All Might to the I-Expo to surprise her father David.
All Might introduces Izuku to Melissa and they quickly get acquainted. It gets uncomfortable for Izuku when Melissa gets close to him to check out his hero costume and she notices the scars on his hand. All Might interrupts and reminds Melissa about their agenda.
Inside David's laboratory, Melissa appears not long after her father mentions her off inventing and reunites him with All Might. She sees David is speechless from the surprise and All Might is very happy to see his longtime friend. Melissa watches as Izuku gets very nerdy student with his knowledge about her father Professor David Shield. David then decides to have alone time with All Might and asks Melissa to show Izuku around the expo. Melissa and Izuku exit through a corridor and continue getting to know each other. Izuku tells Melissa to call him Deku even though she finds the nickname strange.
Melissa takes Deku through the I-Expo where they notice many Pro Heroes including Godzillo, all invited to the event by sponsors. Everyone is also invited to a reception party at the end of the night. Melissa asks Izuku if he's attending and he confirms it, to her joy. Melissa then takes Izuku to the pavilion and shows him tons of different inventions out on display. Izuku eventually asks about Melissa's dad and she admits she's aspiring to be a great scientist just like he is. She is a third year student at I-Island Academy, studying to be a scientist in the same way Izuku is for becoming a hero. They share the same goal of becoming like the people they admire.
Very soon, Ochaco Uraraka, Momo Yaoyorozu, and Kyoka Jiro suddenly interrupt and suspect Izuku is on a romantic date with this girl. Embarrassed, Izuku explains that Melissa is just showing him around the expo. Melissa almost brings up All Might but Izuku stops her and asks her not to say anything. The five of them get tea together at the café. Izuku is relieved at having avoided spilling any secrets. Soon she meets Denki Kaminari and Minoru Mineta who are fascinated by Melissa's beauty while she learns they are classmates of Izuku working as extra staff at the café.
Melissa then meets another one of Izuku's friends, Tenya Iida who runs in a top speed to remind Denki and Minoru to represent U.A. as good workers. Melissa promises to show everyone around once the expo is fully open.
When seeing Katsuki Bakugo and Eijiro Kirishima do the villain attraction, Melissa asks Izuku if the are also his friends, something that he confirms. She then watches Izuku take on the Villain Attack attraction next and manages to score second place with a time of sixteen seconds using Full Cowl at 5%. Melissa notices that Izuku's destructive power is very similar to All Might's, although he's limiting his power. Ochaco and Tenya praise Izuku for his performance, further annoying Katsuki. A U.A. student makes another surprise showing. Shoto Todoroki clears the event in fourteen seconds and steals first place using his ice. Aggravated to the point of no return, Katsuki snaps. He aggressively at Shoto and interrogates him. Shoto reveals that he was invited to represent Endeavor and mostly ignores Katsuki's rude comments. Tenya runs over at top speed again, yelling that Class 1-A can't misrepresent U.A. to the world. Melissa watches in amazement of all these young heroes around her. She commends the U.A. students for their amazing talents.
Melissa and the others regroup with Denki and Minoru. Melissa prepared invitations to the party of them and moves them both to tears. Tenya reminds everyone to meet up at the party at 6:30pm. Everyone disperses but Melissa asks Izuku to join her at her lab. Katsuki refuses to go to the party because he's unprepared but luckily Eijiro brought clothes for him. All Might shifts into his muscle form and leaves David's lab, promising to see him later at the party.
On the campus of I-Island Academy, Melissa shows Izuku to her lab. He notices all her awards and commends her. Melissa claims her grades were slipping but she still studies hard to become a hero. Izuku asks if Melissa wants to become a Pro and she reveals that she can't because she's Quirkless. This shocks Izuku because they were both born without Quirks. Melissa is okay with being Quirkless because she can still be a hero that supports others like her father.
Melissa gives Izuku an old support item she invented that was inspired All Might. She calls it Full Gauntlet and she equips it to Izuku's injured hand. Izuku activates the item and it takes shape as a gauntlet around his arm. Melissa could tell from Izuku's scars and his performance at the attraction earlier that his Quirk was too much for his body to handle. She gives it to Izuku because it can take the full power of All Might's punches three times before it breaks. In other words, Full Gauntlet will allow Izuku to use 100% of his power for three smashes. Melissa entrusts this item to Izuku and asks him to become a great hero that can protect people.
That night, Melissa meets up with Izuku and the others in a dress, stating she hoped to bump into along the way.
All Might's speech is abruptly interrupted when the I-Island Security System sets off an alarm across the entire island. Melissa and the U.A. students are also at the mercy of the security system. Their cell phones have no signal and their access to the elevators have been denied. Izuku tells everyone that All Might is at the party so that's where they need to go. At the hostage situation, Wolfram promises to release everyone in time. He silences a mouthy hero and receives orders to take in a researcher. He attempts to take in Samuel Abraham but David saves his assistant. Both of them are taken in as All Might watches in bondage.
All Might notices Izuku flashing light at him from the third floor. Kyoka uses her Quirk to listen to All Might as he explains the situation. Tenya suggests that the students escape since they can't legally fight villains. However, Melissa reveals that I-Island's security is on par with Tartarus's. Shoto and Kyoka believe that they should try to help while Minoru and Momo believe they can't. Awkward silence is interrupted when Izuku says he wants to save everyone.
Minoru believes Izuku hasn't learned from the U.S.J. Incident, but Izuku reveals they can save people without fighting. Melissa believes in him and tells everyone they can reset the security system if they reach the top floor of the tower. Everyone decides to work together to disable the system so that All Might can defeat the villains and turn the tables. Izuku tries to tell Melissa to stay behind because she's Quirkless. Melissa refuses because she's the only one who can disable to the security system. She wants to help save everyone just as much as Izuku.
Melissa and the U.A. students scale over fifty floors until they run into a shutter cutting them off from the 80th floor. Shutter doors close in on the students from both ends of a corridor. Shoto uses his ice to keep one of the doors open long enough for Tenya to open a path to the Plant Factory. The group notices the elevator on the other side of the floor is going up. They decide to hide and hope the villains don't notice them when they exit the elevator. Two villains do notice some kids, but they turn out to be Katsuki and Eijiro.
Melissa's group don't see a way forward but he notices a small door on a nearby roof panel. Melissa explains it leads to the maintenance room but can only be accessed from the inside. Momo creates a small bomb to open the door and tells everyone they can access still use it to access the outside wall. The group volunteers Mineta for the job and after he accomplishes the goal, Melissa praises him on his bravery, leaving pleased.
In the control room, the villains are irritated by the student's resistance. They demand the invaders in the field deal with the students. The villains fighting Shoto and Katsuki prove to have strong Quirks. One is a beast shifting Quirk while the other allows him to hollow out space. Katsuki defeats the beast with his Howitzer Impact move. He gets his sweat on the other villain, giving Shoto the opening to detonate it with his fire. After defeating the villains, Shoto and Katsuki check up on Eijiro. He's fine, just stuck in place by his own Quirk. They're interrupted when a mass of security bots flood the plant factory.
Melissa's group passes the 100th floor and Denki notes how the shutters have opened for them. Ochaco deduces that the villains are trying to lead them somewhere. They reach the 130th floor and are greeted by dozens of security bots. Melissa watches Momo prepare a sheet of isolation for the group and they execute Plan A. Tenya uses his speed to throw Denki at the bots where the electric hero uses Indiscriminate Discharge 1,300,000 volts. Melissa notices robots are able to defend against his Quirk so Denki turns up the voltage to two million. His power quickly fades out and Denki shorts out his brain.
Wolfram's subordinates notify him that the students got away and he deduces one of them has a sensory Quirk. The students make it to the server room on the 30th floor where they are ambushed by even more security drones. Melissa warns everyone they can't afford to damage the server room by fighting. Tenya and Momo offer to hold off the drones while Izuku takes Melissa on another route. Ochaco joins them and leaves everyone else to fight. Tenya takes on the bots using Recipro Burst while Momo creates a cannon that fires adhesive material.
Izuku, Ochaco, and Melissa continue ahead as their friends fight. The bots manage to outlast Tenya's time limit and Momo and Minoru's bodies have reached their limit as well. The robots use restraining bonds to capture the kids. Izuku's trio reaches the wind power generation system where they hope to avoid getting ambushed again. Melissa asks Ochaco to use her Quirk to float she and Deku toward an emergency exit at the top of the tower.Ochaco lifts Izuku and Melissa toward the exit, but this leaves her exposed to another robot ambush. Melissa tells Ochaco to release her Quirk but she refuses. Just before the robots get to her, Katsuki makes an explosive entrance and saves her. Melissa is relieved to see Shoto and Eijiro join the battle and repel the bots using their Quirks. Wind threatens to carry Melissa and Izuku away but Shoto acts quickly. Katsuki moves one of the wind turbines toward Izuku and Shoto uses his flames to push hot wind in their direction, setting them back on course.
Izuku and Melissa recover from the fall and neither of them are injured. Swordkil suddenly attacks them both and Izuku grabs Melissa and rolls out of danger. Swordkil keeps up his attack using his Sword Hands Quirk but Izuku parries using Full Gauntlet. Swordkil nearly gets the better of Izuku but Melissa helps him. The villain knocks Melissa aside but this gives Deku an opening to land a smash to defeat Swordkil. Izuku makes sure Melissa is okay before they continue their mission.
Izuku fights his way through more of Wolfram's henchmen until he and Melissa finally reach the top floor of the tower. Together they find David Shield inside some sort of vault. Inside the storage room, David manages to unlock a special block and sends Sam to retrieve a special item. David swears to never let this device into the wrong hands and Sam replies that everything is according to plan. Melissa overhears the men talking about working with the villains.
Melissa makes her father admit he was working with the villains to get the invention that was taken by sponsors. It was originally Sam's idea to hire fake villains to steal the research back for them. Izuku and Melissa are both horrified by this revelation and the latter demands to know why. David admits that he was doing this for All Might because his Quirk is disappearing. Izuku feels guilty that All Might lost his power and now David is acting desperately on that. Angry, Melissa tells David that she and her friends risked their lives to save everyone.
David is confused on her statement and says the villains were supposed to be fake. Wolfram makes a dramatic entrance and reveals that the only act was the villains pretending to be fake. Izuku tries to attack but Wolfram quickly restrains him using his metal manipulation Quirk. Sam betrays David and brings the device to Wolfram. He claims that David turned his back on him when they never got paid for working so hard. Wolfram shoots Sam in the arm and acts like they never had an agreement. He shoots again but David jumps in the way of the bullet.
Melissa is horrified to watch her father tortured by Wolfram and pleads with the villains to return her father. Her cries fall on deaf ears and Wolfram threatens to eliminate her. Izuku breaks free from his bonds and attacks, only to be blocked by Wolfram's Quirk.
Izuku tells Melissa that he'll save Sam and David but she needs to be the one to say everyone else. Melissa flees and Daigo tries to follow. Izuku refuses to allow him to pass, forcing him to come to blows with Wolfram. Together, Melissa and Izuku manage to save everyone by disabling the security system. All of the drones return to normal mode and I-Island is set free from the grasp of the villains. Wolfram prepares to escape with David on a helicopter and Izuku pursues them. Melissa calls All Might to update him on the stakes. He promises to rescue young Izuku and David.
Melissa watches at Izuku mirror's All Might's power against the greatly enhanced Wolfram and is moved to tears by their teamwork.
Wolfram's last-ditch effort is forming his metal into a massive iron cube large enough to crush the tower. All Might and Deku go beyond their limits and break through the cube with a Double Detroit Smash! David and Melissa watch as the U.A. students cheer on their heroes to victory. All Might and Izuku chant the time long motto of U.A., "Plus Ultra", as they finish the villain off with twice the full might of One For All.
At last, the heroes are victorious and the scrap metal hell disintegrates into dust. Melissa checks on Izuku and All Might on David. David thanks All Might for saving him but All Might bestows the credit on their successors. They are the two heroes who saved the day. Melissa appreciates everything Izuku has done. Izuku reminds her that her actions helped save everyone on this day.
All Might tells David that Melissa explained the details of what happened. David admits that he was so afraid the Symbol of Peace would fade away and lead to the end of peace. David realizes now that he was just trying to hang on to the past, and knows that Izuku and Melissa will protect the future and ensure peace to everyone. All Might says Izuku isn't ready yet but he possesses the potential to be the greatest hero. David says he can feel the same light in Izuku that comes from All Might, the light of a hero.
Sometime later, Melissa visits her detained father in the hospital. They speak as he recovers from his injuries.