It's lovely. It's about a 30-year-old man (Daikichi) who finds himself getting in the bath with his aunt every day, sleeping with her and acting as if he were her father.
It's entirely innocent, by the way.
I'll explain. At his grandfather's funeral, Daikichi learns that the distinguished gentleman had been enjoying a more vigorous retirement than his family had suspected. Grandad had a girlfriend (identity unknown) and a illegitimate daughter. The girlfriend's disappeared for reasons of her own, leaving the family with a quiet, withdrawn problem called Rin.
Just to recap, here's a family tree. Each line represents a new generation.
(a) The Old Rogue (79), deceased, great-grandfather
(b) Daikichi's mother (presumably late fifties) and her half-sister Rin (6)
(c) Daikichi (30) and other siblings/cousins, all old enough to be the parents of...
(d) Rin (again), who's the same age as her great-niece Reina
Rin's a nice kid, although she's not good with strangers. At the beginning, that's everyone. Grandad might have been the only person who knew she existed. I'm not sure that was best. Mind you, I was being harsh calling grandad an "old rogue" and it sounds as if he was a sweet-natured old gentleman. Rin certainly loved her father and never forgets him throughout the series. She'll talk to the grave when they return to pay their respects and no one's more sensitive to what he'd have wanted during his funeral.
All this can make Rin a bit odd. She's a normal six-year-old who'll sometimes talk like a great-grandmother. She thinks mere grannies look too youthful to be called "granny" (which they find hugely flattering). Anyway, Daikichi loses it a bit when the family starts having a Someone Else's Problem discussion on what to do with Rin. The rest of the series is about an ugly, mildly antisocial batchelor discovering what it's like to be a surrogate father.
The obvious point of comparison (for me) is with Yotsuba &!. Both are slice-of-life shows about a very small girl and her imperfect father-figure who's not really her father. The main difference, I think, is that this show is Daikichi-focused. It's about what it's like to be a parent. Daikichi resigns from his job to take something more menial that doesn't demand gruelling overtime. He has to organise nurseries and school places. (That nursery he finds is surprisingly forgiving about parents arriving late to collect their children, by the way. Don't expect yours to be like that, either in Britain or Japan.) He discovers that children expand your social circle, including one smoking hot single mother whose brattish son appears to become Rin's best friend. (Daikichi behaves himself, but even so they're practically a family of four by the end.)
We also see that it's not a bed of roses. Parents have to give up a lot, e.g. their entire lives. Free time? What is this thing called free time? Single parents have it tough, but being married isn't the magic answer either. Ep.7 gives us a brutal view of what Japanese married life can be, to the extent that some Western viewers have had trouble with some of its messages. "As long as I try not to feel anything, I should be able to hold out, more or less." (Mind you, Haruko's not being painted as perfect either. She's a snippy bitch to her husband, in the short time we see them together.) The scene with Daikichi's sister talking about potential marriage has the potential for East-West values dissonance too, but all this and more is a reality for many Japanese people.
Yotsuba and Yousuke are a lot more laid-back than Rin and Daikichi, who always seems to be struggling just to keep up. (Ironically he ends up giving the impression of being super-dad, given how much effort he's obviously putting in and how much he worries about Rin.) As for Rin, she's serious enough that she often appears to be Daikichi's mother. She often does the cooking. She'll correct his table manners. As for being friends with Kouki (the irredemable Small Bad Boy), she's not so much his mother as his minder. However she blossoms over the course of the series, going from practically a ghost (ep.1) to being far more outgoing and even a chatterbox (ep.11). She's also often very girly. (Perhaps significantly, the original manga is written for women by a woman, whereas Yotsuba &!'s manga-ka is male.)
Aesthetically it's a pleasant series. The pace and tone are soothing, as are the carefully crude but attractive watercolours. I appreciate the fact that Daikichi and even Rin aren't generically pretty (although Rin has her moments), while Rin's voice actress isn't doing a cutesy child voice. Oh, and the charming theme song is by Puffy AmiYumi (or, as they're known in Japan, just Puffy), doing something you'd never have expected from them given their back catalogue. (The song is apparently also used in Sabu's live-action movie, which makes me happy, and Ami and Yumi even get voice acting cameos in ep.9.)
Warning: the manga is a bit different. The anime's only doing vols.1-4, whereas vols.5-10 span a decade of the characters' lives and end controversially. However that's the manga and this is the anime. It should be possible to watch one without reference to the other. (I believe Sabu's film also follows the anime's lead, thus making the manga the outlier.)
This is a heartwarming show. Everyone means well. Some of them have problems and they can be selfish or thoughtless (e.g. Rin's mother), but it had to be that way since the show's taking a fairly hard look at some of the realities of parenthood. It also gets funnier the more Rin gets of the spotlight, although of course there wouldn't be a show without her. I'm terribly fond of it. It's quite low-key, but it has some lovely moments and episodes, e.g. the surname discussion or the tree. Wholeheartedly recommended.