While I will try to keep this spoiler free the spoiler warning is in effect.

The world is enraptured by a sensational new pop idol called Uta who has become a symbol of hope for all the regular people caught up in the pirate age, the Strawhats being among the crowd.
Luffy upon seeing Uta, immediately recognises her and jumps up on stage, revealing to the entire world that she is the daughter of Red Haired Shanks.
Pirates want to kidnap her to get to Shanks while the Marines think her influence is way too powerful to be allowed to live.
Uta with her songs are able to turn the tide against any assailant, even Big Mom's pirates and she is able to have some valuable catch up time with Luffy.
But once Luffy brings up that he's a pirate, Uta's demeanour changes as she takes Luffy's crew away and demands Luffy quit being a pirate using ever more violent means, it starts unravelling after Trafalgar Law and Bartolomeo step in to save Luffy that the situation they are in is more dangerous than they originally thought.
You can tell immediately that this was a joy to work on from the animation right down to singer Ado's incredible performances making for an absolute flawless cornucopia of colour and music.
The film isn't as high on the epic set pieces set by the last One Piece movie Stampede but it more than makes up for it with Uta being the most twisted villain One Piece has ever made equal only to Doflamingo and its Uta's ever deteriorating mental state that makes the songstress even more terrifying in context.
I deliberately didn't watch the dub as the song numbers blend so much better in a Japanese dub track than the awkward switch between Japanese songs and English speaking dialogue.
Final Verdict: The lesson here is never bet against One Piece, even if you think they are passed their best, they keep surprising you with more spectacular stories and for what looks like a fan fiction plot is actually the best One Piece movie to date.