The original manga was just a modest eight volumes in 1992-95, but it has ten million copies in print and has since spawned a 76-episode anime series + movie (which I've seen and would recommend), a light novel series, a 2001-02 Taiwanese live-action TV series and now this 2018 live-action movie. That's a moderately big deal. The version I know is charming and lots of fun. I'd been looking forward to this film.
It's not actually that bad and I genuinely like some aspects of it, but it's a failure. It lacks charm, partly because of Hinako Sakurai in the lead role as Miki. Also, somehow, it's managed to take idiot plot points that I used to forgive and make them look hysterically terrible.
The story involves Miki, Yuu and their parents. One day, the four parents announce that they're getting divorced to remarry each other! Yuu's father will marry Miki's mother, while Yuu's mother will marry Miki's father... and then everyone will live together in one house with their gobsmacked children. Hurrah! What's more, the parents leaped into all this on holiday in Hawaii. (There is actually a hidden reason for all this, but the culprits keep it secret and so one's imagination runs wild. You'll probably assume that some wife-swapping went too far, making one wonder if the musical bedrooms might continue afterwards. Infidelity is almost a theme in this movie.)
All this is the best part of the film. The parents are appalling. You'll cringe. If you were Miki, you'd be eating your own leg to escape.
Unfortunately it soon becomes clear that Miki isn't just being grumpy at her four "parents". She's just grumpy, full stop. The performance works and is logical, but she's a pain in the arse. I wouldn't even call her likeable. You'll almost end up siding with her parents after she's finished bitching selfishly at them. It's also a bad look, making Hinako Sakurai look like a sullen goldfish. (She's pretty when she smiles.)
Frankly, I was more invested in the supporting cast's relationships. (They get very little screen time and they're a sad shadow of their anime selves, but that was probably unavoidable. Ginta is sabotaged by script-enforced stupidity, but I cared about Meiko's subplot and about the delicately tragic Arimi.)
Ryo Yoshizawa is good as Yuu, though. He's helpless in the face of the dumb script in the second half, but he's good and he manages to sell the character's well-hidden broken bird syndrome and bad judgement. The four parents do well too. This is basically a well-acted film. Sakurai is the only weak link and even she's perfectly competent, but making unwise choices.
It's a film of two halves. The first half ends with a mutual love declaration and is quite good, with only a moderate amount of stupidity. The second half roars off into Painfully Stupid Land and is a train wreck, except at the comedy gold ending. You'll be roaring. Your eyes will pop. (And not in a good way.) The problem, I think, is that the anime was a 76-episode melodramatic soap opera. You accepted crazy stuff and insane failures to communicate, because... well, that was the genre. The show had nailed its tone. This, though, is a live-action film with a realistic tone and a comparatively dour heroine. There's nothing to make you forgive stuff like:
STUPIDITY IN THIS FILM
1. Once upon a time, Miki confessed her love to Ginta. He said "no" for a dumb reason, then dumbly hid the fact that he'd always been in love with her. Now, though... "If I win this tennis match, go out with me!" (What's a tennis match got to do with anything? Oh, and our heroine doesn't even bother to give him a response. That evening, she snarls at Yuu when he tells her that she owes Ginta at least a "yes" or "no".)
2. Yuu discovers a photo that makes him suspect that he might be the biological son of Miki's father. They've already fallen in love and are planning a trip together. Does Yuu show Miki the photo? Does he explain? No, he just tells her he's lost interest in her and runs away to university in Kyoto. Naturally, Miki has no way of guessing the situation and is miserable for months, not knowing what went wrong and having no way of stopping herself from loving him.
Yuu is an idiot and a bastard. You can see how this behaviour fits into his issues, but yeeeesh.
3. Eventually Miki hunts Yuu down and confronts him. Obviously the outcome has been exactly what he didn't want. D'oh. At last he shows her the photo. This makes Miki annoyed at their parents for not having talked about all this earlier. Not Yuu. Their parents.
4. Miki: "let's contact our parents and find out the truth!" Yuu: "no, let's not."
5. Yuu and Miki fall in incestuous love. (Regardless of whether or not it is incestuous, they believe it is.)
6. They go home and tell their parents they're getting married, even though they're blood siblings. By this point, you'll be on the floor in pain from laughter. It then gets even sillier. Remember, kids, bad communication kills. (No one dies, though, except you in the audience.) Note that Yuu had been assuming he'd been conceived before his parents got married, but was born two years after the wedding. Couldn't he count?
One-line summary: JESUS WEPT. The first half is quite good, although also a bit leaden thanks to Miki. The film then changes direction in a rather disjointed, episodic way and becomes arrant nonsense. You'll want to strangle Yuu, or perhaps the scriptwriters. The film could arguably be read as an exploration of questionable sexual behaviour and its consequences, in which case you could read mischievous meaning into the pointless final scene of everyone passing around the marmalade and getting their lips around it. (Yuu is the Marmalade Boy of the title, as named by Miki.)
(For what it's worth, Wataru Yoshizumi had originally been going to have Yuu and Miki really be blood siblings in the manga. Her editor talked her out of it.)