Fans who want to see more of All Might's power will definitely have a good time here as the film takes place before Season 3 of the series as Horikoshi teases more scenes featuring All Might's One For All power.
Horikoshi states in his most recent interview that he was VERY INVOLVED with the Boku No Hero Academia movie’s script, characters, plot, what he wanted to show in the movie, what he wanted characters to do in the movie. etc. Here’s the translation!— ☆オードリーAudrey☆ (@aitaikimochii) July 7, 2018
Source: https://t.co/8ZRV0thhoj pic.twitter.com/rhKnAuLCbp
When asked about the film being set before All Might's battle with All For One, Horikoshi confirms that the film is allowed to explore a new move with Deku and All Might because All Might has yet to lose One For All.
"Yes, the events of the movie happen before the battle with All For One, so I want everyone to be able to watch and look forward to this since you will never see their duo move in the manga."
In the third season of the series, All Might has a huge battle with the villainous leader of the League of Villains All For One. The resulting fight leads to All Might losing his power, and thus meaning the movie will be one of the final times fans will get to see All Might's All For One in action.
If fans were doubting whether or not the film would tie-in back to the series' canon, seeing Horikoshi's involvement and placement into the series' timeline further emphasizes the "canonicity" of the film. The fact that he and Midoriya will have a dual smash attack at some point during the film is just icing on the cake.
"After the end of the climatic Final Exams, the U.A. Class members head off to their school field trip for Summer break. However, before they head off to the school field trip, All Might and Deku accept an invitation from someone to go abroad to a floating and mobile manmade city, called 'I Island.' All researchers in the world, from both East and West, are gathered in this place, known as the Hollywood of Science, where they research quirks as well as hero supplemental items at the special 'I Expo' convention that is currently being held on the island.1comments
This is where Deku meets the quirkless girl, Melissa. Melissa also is like him, quirkless. Deku is able to connect with Melissa by recalling when he too was quirkless. During that time, suddenly, despite an iron wall of security surrounding the island, the system is breached by a villain who hacked into it! The people living on the island are all taken hostage! Now, the Hero Association needs to act quickly to deploy their unshakeable 'Plan' or else―! The only person who can grasp that key, is none other than the Number One Hero, All Might!"
For those unfamiliar with the My Hero Academia juggernaut, the series was created by Kohei Horikoshi and has been running in Shueisha's Weekly Shonen Jump since July 2014. The story follows Izuku Midoriya, who lives in a world where everyone has super powers but he was born without them. Dreaming to become a superhero anyway, he's eventually scouted by the world's best hero All Might and enrolls in a school for professional heroes. The series has been collected into 15 volumes so far, and has been licensed by Viz Media for an English language release since 2015.