Sorata Kanda is an ordinary schoolboy in Sakurasou Boarding House, a dumping ground for nutters and geniuses. They're all still at school, but... Misaki is already rich from her earnings as a brilliant professional animator and is also a hyperactive loon with zero modesty. Jin is a writer with four or five girlfriends, some of whom are married. Ryuunosuke is a computer programmer with extreme personality issues who rarely leaves his room and doesn't go to school. Sakurasou also has a live-in teacher (Chihiro) who theoretically oversees its students, but she's a lazy, man-chasing drunkard.
Ep.1 then introduces yet another resident, the eponymous Pet Girl. Her name's Mashiro Shiina and she's probably autistic. She doesn't understand social cues, how to express emotions or the significance of underwear, but she's a world-famous artist. She can also get 100% in school tests, despite having no understanding of their content, by memorising a crib sheet as a picture.
Sorata gets unanimously (by everyone except himself) nominated as Mashiro's minder. This is a thankless job that definitely shouldn't be done by a boy, but no one cares about that except Sorata himself. (He appears to have suppressed his sexual desire and will yell at his female housemates if they dress, talk or behave inappropriately, which is often.)
This is a strong but uneven series. Its primary theme is probably the stress that's created by a talent gap. You've got people like Sorata and his classmate Nanami, who are trying to pursue artistic goals without having that much talent for it. They're serious and they'll try really hard, but the show's downright brutal in how it'll smack them hard with failure. Sorata in particular is capable of lashing out in pseudo-retaliation against whoever happens to be standing in front of him, even though it's not their fault. What makes it ten times as hard for them, though, is the presence of airheads like Misaki and Mashiro, who do it all as naturally as breathing and don't even seem to know what failure means.
The geniuses are a mess too, though. Misaki is desperately in love with her childhood friend Jin, but can't get through to him and doesn't know how to. (Sorata's jealous of Jin's success, but Jin knows he's below Misaki's level.) Mashiro is a prisoner in her own head. Ryuunosuke alienated all his friends by treating them like his inferiors (which in terms of talent they were) and is now a hostile, bitter recluse.
This show is usually funny and warm. It has a sharp, uncomfortable edge, but the cast are charming and the jokes are funny. Mashiro has a knack for saying true things in the most misleading way possible, which can be explosive when she doesn't understand nudity and has been known to crawl into Sorata's bed. My favourite episodes were the ones with Sorata's sister, Yuuko, who might be an overreaction machine with a brother complex but she still understands the house's relationships better than her brother does. (See ep.7 and eps.15-16.)
That said, though, I hated the first half of Season 2. It's bad enough to lower your opinion of the show's writing as a whole and I'm not surprised to hear that the original light novels went down the toilet in later volumes. (This anime adapts the first six of ten novels.) Every male character in eps.13-18 of the anime becomes cruel to women who love them, as a result of self-obsessed emotional issues that are understandable up to a certain point only. After the climax of Season 1 and its de facto love declaration, Sorata's rebooted to a bitter, hostile person who's nasty to Mashiro because of stuff in his head. I hated him. (He gets over this quickly and we're supposed to be happy about the reconciliation, but the damage remains and Mashiro's walking on eggshells with him for the rest of the season.) Ryuunosuke is a shit to Rita in ep.17. Yeah, he's Ryuunosuke, but still.
Worst, though, is Jin. He hurts Misaki badly, for reasons that are understandable but don't justify his actions. Emotionally, she's an open wound. She's in despair and she doesn't know what to do. This goes on and on. It's unpleasant. This builds up to a reconciliation that again we're meant to see as heartwarming, but Jin didn't deserve it. He wanted dumping, ideally down a mineshaft.
Next, there's a story arc about the school wanting to tear down Sakurasou. The residents spend all their time collecting signatures for a petition, even though it's mathematically impossible for them to get enough. (They need two-thirds of the student body, but it's the end of the school year and the third-years have stopped going to school. In other words, they need 100% of the first and second years. Not happening.)
Thanks to that run, I'll be ditching these episodes. I hated eps.13-18. It's still a strong show, though, and definitely worth a look. It's often very funny, but it's pulling no punches with its characters' flaws. The central love triangle can hurt. I love Mashiro, Misaki, Nanami and even Yuuko. I'm also meant to love Sorata, Jin and Ryuunosuke, but... well, I can respect the courage of the portrayal of them. They are good people, though. (Even if you sometimes need to remind yourself of that.)