The film adapts the final 4 Volumes of the Quintesstial Quintuplets Manga and finishes out the Storyline of the Series, including the Reveal of who Fuutaro Uesugi Marries in the Flash-Forwards.
In the Manga, the Pool scene that opens the film has a lot more Detail. Fuutaro invites the Quintuplets to the Pool after they miss out on going on a School Trip to the Beach, playing a somewhat vital role in his realization of his feelings for them. The Pool Scene itself has dialogue in the Manga, but the Film acheives the same scenario through Visuals alone.
In the Manga, Ichika's reveal that she plans on Dropping Out of school to further her Acting Career is much a more detailed Storyline; while in the Film it's merely mentioned in passing due to Time Constraints in order to explain her absence in much of the Festival arc.