Chiikawa looks drawn for (and possibly by) 4-year-olds. Its happy, friendly characters have heads that are 80% of their bodies. Chiikawa itself is probably a mouse rodent chinchilla thing and his friends include things that are cat-like, a rabbit and an Okinawana mythical guardian lion. The audience at the Japanese cinema where we saw this was mostly children.

Appearances are deceptive. The original manga has dark humour and an eye to an adult audience, while some of this film's characters want to kill and eat each other. Sometimes, they manage to. There's a Japanese legend that says that eating a mermaid's flesh grants immortality (also used in Rumiko Takahashi's Mermaid Forest manga and anime)... and this film based its plot on that. It's looking at revenge, murder, guilt and eternal torment. Social media users have called it a horror film, while young children have been known to leave cinemas in tears. It's also been compared to the work of Kanae Minato, a Japanese crime fiction writer best known for Confessions (which involved a schoolteacher seeking revenge for a dead child and made the shortlist for Best Foreign Language Film at the 2011 Oscars).

Result: it's the biggest film in Japanese cinemas so far this year. It's taken over 13.9 billion yen (90 million dollars). When we were driving to the airport, our taxi driver was talking about it and said he'd seen it in the cinema with his university student son. (He said it looked like a children's film, but by the end definitely wasn't one.) People in Hong Kong were wearing Chiikawa T-shirts.

Our Chiikawa heroes visit an island populated by creatures even more crudely drawn than themselves. These talk in cute noises (although they can sing in Japanese), they have no faces and they look like grey crystal balls with little skirt sections. They don't even look sentient at first glance, but they can perform heroic sacrifices, get eaten by a Siren (actually a giant cat-mermaid) and agree to kill anyone who threatens their community. Also, on becoming immortal, they grow battery packs in their backsides as if they'd become robots. That was weird. Meanwhile, the cat-mermaid is basically just doing what cats do, except that its victims could be intelligent. Oh, and one of Chiikawa's associates (i.e. the heroes) is a squirrel who's full of himself and gets enthusiastic about eating a mermaid himself on hearing about the legend.

The movie's first half is fairly close to what the art style would suggest. The second half goes in unexpected directions and hits sincere emotional notes (from nearly mute faceless grey sphere beings). Also, there isn't a villain. Everyone's actions grow from understandable motivations and from what had been done to them by others.

There's also a character named Momonga, but that'll be the Japanese dwarf flying squirrel rather than the mass-murdering villain protagonist of Overlord.

Is this film family-unfriendly? It would seem not. This was the third time Misaki (8) and Natsuki (13) had seen it, even though we'd suggested watching something else instead. They like it. Besides, the cinema was indeed full of children. I'm pretty sure English-language cinemas wouldn't know what to do with it, though. It genuinely is the jolly, simplistically drawn film for small children that it looks like... but at the same time it's really not.