A centuries-old skull-headed magical thing (Elias) buys a fifteen-year-old girl (Chise) at a slave auction in modern London, while she's only wearing a shirt and a collar with a chain. He plans to make her his wife. This is their strange story.
That's a problematic premise, to put it mildly, but it has some wrinkles. Crucially, the person who put Chise up for sale is Chise herself. She gets the money. She wanted to become property, with absolutely no conditions put on the seller or on their intentions for her. The phrase you're looking for is "frighteningly unhealthy", or possibly "self-loathing". When the show starts, she's an emotionless dead zone whose mother committed suicide after trying to kill her and wishing she'd never given birth to her. Chise then had an upbringing of being bullied by her host families because she kept being terrified of supernatural beings no one else could see. Selling herself into slavery wasn't the worst of the options she was thinking about.
Oh, and she's a Sleigh Beggy (or Slay Vega), which is one of many, many examples of a detailed use of Celtic mythology, some of it pretty obscure. The show draws on other sources too, though. Early episodes reminded me to varying extents of Shakespeare (A Midsummer Night's Dream), Harry Potter and even Lovecraft (The Cats of Ulthar).
As for Elias, he's a super-old half-faerie bone thing with such a poor understanding of humans that he'll be learning about emotions from Chise. Talk about the blind leading the blind. He was goofy-cute when he was a little bone thing, but even so he's capable of being petty, jealous, childish and/or having a monster's view of the value of human life. He won't even understand that he's doing that. He's like a very polite Cyberman that's bad at analysing its own unexpected responses. He'll do things like offering to erase your memories. That said, though, he's calm, respectful and supportive. He tries to do the right thing. He genuinely does want to help Chise, even if his motivations might be off-putting, and he never behaves as if he's her owner (even though he is) or deliberately does anything that he thinks might make her uncomfortable.
I found the first few TV episodes the least engaging. Chise and Elias haven't yet got used to each other and there's formality rather than warmth in their relationship. Gratitude, sure. (Chise latches on to Elias like a barnacle simply for accepting her.) I think it's ep.8 before she even manages facial expressions, but she's definitely lost her emotional distance by the halfway point.
Ruth improves the show. He's a Black Dog (as per folklore all around the UK) and he pulls episodes in a more human direction, even when he's usually not that important. I also liked ep.9, in which Elias has stayed in his room for two weeks without eating. Chise worries about him, while Silver worries about Chise. That's a important step in the show's journey towards more warmth. This is a mature, intelligent, gently paced show that's not afraid to spend a lot of time on detailed character scenes. That's a great thing when we're invested in the characters, although it could have become sleep-inducing if we weren't.
It's a show that's both gentle and dark. It has faeries, but they're taken from folklore and so you need to be very, very careful around them. Mind what you say in their earshot. Watch out if you're offered food. There are dragons, but they'll bring in issues of racial extinction, child abduction and embracing one's mortality. There's a village priest with the job of keeping an eye on Elias. (They're sort of friends.) There's a banshee and a vampire, although neither acts in the traditional manner.
Aesthetically, the show's top-notch. I live in London and I used to live in the English countryside. This show's rendition of both looked, to me, both flawless and beautiful. They've created a luscious animated world. The only oddity is the occasional super-busty female character with a neckline that would make almost all women topless. You'd only be safe if your nipples were on your underboob. (Mind you, that's straight from the manga, which has a female writer/artist.)
This anime might blow your mind if you're a folklore buff. If you're not, you'll be watching for the mature writing and the character development. Elias and Chise are very likeable, but they're also both emotionally stunted and in desperate need of a ton of growth. They're capable of making dangerous mistakes. Elias can make a five-year-old look mature, which isn't good from an otherworldly monster. There's a lot of development in this series, as it follows two stunted emotional pygmies with their own special problems (suicidal self-worth issues, non-human morality) who started their relationship by moving in together. They are good people, though. Sort of. Even if it's just learned behaviour for Elias, which he's capable of falling away from.
It's the kind of show that's obviously excellent, even if it's not your cup of tea. The detailed animation, the careful pacing, the research, the character development... it's the kind of thing you might put on to impress people who don't like anime. It's hardly the cuddliest show out there, but it's clearly of high quality. I enjoyed it, for what it's worth.