It's based on a children's book series about twelve-year-olds solving crimes, like the Famous Five but in Japan. I enjoyed it. The first story arc asks us to accept a silly level of forensic expertise from our heroes, but otherwise it's a fun, harmless, surprisingly decent little show with likeable characters.
The heroine is Aya Tachibana, who's a bit more complicated than she needed to be for this kind of story. When ep.1 starts, she's got no friends. "I've always been really careful in front of people." She actually has to learn about not pushing people away. What she hates above all is standing out from the crowd, which means she's going to be mortified when her fellow detectives (i.e. hot, popular boys) start coming to her school to meet her. Ep.5 is mostly about Aya's self-inflicted social failures on starting middle school. That said, though, she's not actually shy and she'll discover that she rather likes having friends. She can be needy. She can lose her temper at the boys. However she's also the sensible, normal one of the group and very engaging.
The boys are simpler characters. Wakatake is loud, rude and an attention-seeker, to Aya's horror and incomprehension. Kuroki is the charmer and networker. Uesugi is a maths genius. Kozuka is brilliant at all things scientific and a bit child-like.
They came together because of their study group. They're all superb at one subject (in Aya's case, languages) but correspondingly weak at others. Put them together, though, and they're quite a force to be reckoned with. That's the show's format. These sixteen episodes are split into four story arcs, each adapting a novel in four episodes.
1-4: THE MISSING BIKE KNOWS
It's establishing the cast and the set-up, so the character development's a bit meatier than usual. It's an origin story. Aya discovers, to her surprise, that she quite likes these boys... but it's still not easy for her and she'll be insecure about her place in the team even in subsequent story arcs. That's all good and this is basically a fun, engaging story, but it includes some silly story points. Most obviously, we're expected to believe that Kozuka has supernatural forensics skills. Give him some paint scrapings and he'll tell you the make and manufacturer of the foreign-made car they came from. Give him some dirt from its tyres and he'll tell you that the culprit lives in Nitta City, shortly afterwards supplying his address.
Similarly, recording one of someone's phone calls made me go "WTF" and I was laughing in a bad way at Wakatake confronting the (adult) villain and kicking a knife from his hand while saying "good thing I play soccer!"
5-8: THE EGG HAMBURG STEAK KNOWS
Wakatake has a pork allergy which flared up after eating a beefburger. He reckons this needs investigation. Surprisingly, he's right. This story also includes a boy (Sunahara) who's damned by everyone except Aya as a delinquent. It also has a surprisingly dark ending.
Kozuka can do DNA analysis of burgers.
9-12: THE BACKYARD KNOWS
Uesugi's acting worryingly out-of-character, but Wakatake once again wants everyone to investigate something of trifling importance to everyone except him. It's a good story. Uesugi's secret I hadn't seen coming. One thing I liked here, incidentally, is that at last the show broke its genre's rules and got the police involved. Until this, it had been coming up with ingenious and satisfying one-off reasons why one child or another didn't want to tell the cops, which was fine as far as it went but surely couldn't hold up in the long run.
13-16: THE VALENTINE KNOWS
Aya's being weird about Valentine's Day chocolates. I'd been wondering why she didn't just give 'giri' chocolates to everyone as is practically compulsory in Japan, but no. She seems to want to give 'em to just one boy and that's that. This is a children's show and normally it has no more than the most fleeting hints of possible romance one day, but this isn't normal. Aya's not within a million miles of starting a relationship, obviously, but she's still getting worked up about boys and Valentine's Day. Furthermore, Sunahara returns. They'd already had chemistry, but here they're steamy. Sunahara's scary circumstances mean that he keeps pushing her away... but he also keeps making telephone love confessions. (Admittedly he doesn't actually say "I love you", but some of his dialogue would have made you expect a marriage proposal had the characters been ten years older.)
I also appreciated a bit at the end where the boys get into a fight. Many stories equate fisticuffs with heroism. This one, in contrast, has some realism. Our heroes are up against a larger group of bigger, more violent boys. They don't magically win and they don't even all participate. The non-physical Kozuka's hiding in the bushes with Aya and no one thinks any worse of him for it.
It's good. Anyone could enjoy it, I think. Kozuka's a bit of a magic wand, but only at the beginning. It might look like a reverse-harem show, but the original novels aren't marketed specifically at either gender (although the manga adaptation is in a girls' magazine). I also like the realistic, low-level crimes that they investigate, without the fantasy and amazing serious crime waves caused by, say, Detective Conan. It feels true to itself. I hope they make more.