As I understand, Hotaru was originally a manga series from 2004, then two television drama serials in 2007 and 2010. The eponymous character is a gimmick unto herself — a Japanese female version of the Italian self-made man who is a slob at home, lacking in all domestic skills, and needs to be pampered like an overgrown child by their significant other — assuming they can get hitched at all. The charm of the Hotaru franchise is the realisation by its creators that this comic character type becomes even more hilarious with a gender reversal. In Japan especially, this gimmick neatly coincides with another of its stock comic stereotypes — the woman left on the shelf.
Hotaru the Movie attempts to reunite its core cast of Haruka Ayase and Naohito Fujiki. It's highly challenging given that at the end of the television serial, Hotaru and her boss ended up marrying each other in what was supposed to be a franchise ender gimmick. Here as a follow up, the couple end up spending their honeymoon in Rome.
The comedy should come from seeing Haruksa Ayase recreate her female slob/overgrown child routine after a long hiatus, and from seeing her character's gimmick evolve in the context of married life — but that doesn't really provide sufficient material for a feature film. The kooky wife and her patient husband attempt to recreate in their wrong genre savvy style the courtship of Peck and Hepburn in Roman Holiday. There's great comic potential in the concept of 'entertainment tourism' where tourists re-create or attempt to experience their favourite films or drama serials on site (see the brilliant Hello Stranger). This might have worked if the filmmakers had chosen that as their main focus.
What doesn't quite work out but takes over the Roman Holiday parody/tribute is a sub-plot involving a Japanese expat who like Hotaru is a loveable slob but has a tragic, dramatic backstory that comes to the foreground and overshadows the entire film. More than a distraction, this subplot is a serious dampener that shows a creative team running out of ideas at times to tell what should be their main story.
That said, Hotaru the Movie should please fans of its long-running drama serials.