Ryoto (Masaharu Fukuyama) and his wife Midorino (Ono Machiko) are in a happy relationship, both well off thanks to his well-paying, time-consuming job. They also have a 6 year old son Keita, whom is smart and talented, but Ryoto seems to be slightly distanced from. Then a bombshell is dropped; the hospital where Keita was born calls and informs them that their son is not actually theirs, having been swapped with another on the day his was born. They meet up with the parents of their actual son, Ryusei, and discover he could not be more different to Ryoto, due mostly to the entirely different upbringing that his parents have given him, which is more loving but less economically stable.
From there, the movie asks the horrific question that could happen to any parent at any time: do you choose your own, blood-related child over the one you have been loving and raising for the past few years? There is a lot going on here for your brain to chew on, as Ryoto and Ryusei's father Yudai (Lily Franky) quietly clash over their differing attitudes to the situation. Yudai continually brings up just how much they'll get in 'damages' from the hospital, seemingly more concerned about that aspect than the well-being of their child. Then there's Ryoto, who believes that just because he has more money, is automatically a better care-giver. This is Nature VS Nurture writ large, and covering every and all aspect you could imagine.
If there is a fault to be found, it's that the film never really tries to wrench out your heartstrings, instead gently plucking at them. With a topic like this, over-the-top hysterics would be a natural reaction, but everyone involved seems quite buttoned-down in their response. Whether this is a cultural thing, or if it was simply how they were directed, either way we're not given the cathartic emotional fireworks one might expect.
However, this is still a staggering display of fantastically realised, fully three-dimensional characters dropped into a terrible scenario, and it is dealt with intelligently and, perhaps most surprisingly, with a certain level of acceptable humour. Plus it leaves you pondering the question that all good cinema asks, 'What would you do if this happened to you?'