It's an anime about medieval economics and the life of a travelling merchant. Kraft Lawrence makes a living buying things in one place and selling them in another for profit. He's the title's "Spice", although as it happens that's just one of various different kinds of goods he might be carrying.
"Wolf" is his travelling companion, an ancient wolf-god called Holo who normally takes the form of a girl with a wolf's tail and ears. She's capable of turning into a wolf big enough to crush houses underfoot, but she doesn't like to, because:
(a) if anyone saw this, they'd scream "witchcraft" and Lawrence would never again be able to do business in the area.
(b) there wouldn't be any point. This isn't a "swords and magic" fantasy series, but one about currency exchange rates, market crashes, the dangers of buying on credit and the equivalent of a futures market. There are no orcs or goblins. Our heroes don't need to kill anyone and this would have been a pure historical if it weren't for Holo herself. It's even historically accurate, with the writer for instance having researched medieval food and clothes. The Pasloe harvest festival in ep.1 is a representation of the "wheat wolf" tradition of medieval Germany and France.
As a curiosity, the show was broadcast in two seasons, each made by a different studio. Imagin made the 2008 season (12 episodes plus an OVA), then Brain's Base took it over the following year. There's not much difference between them, mind you. They're adapted from the same light novel series, one continuing from the other, and they share the same director, scriptwriter, composer and voice cast.
Anyway, if you're in the right mood, it's quite nice. There are two reasons to watch it, one being the medieval economics (which can be a bit dry, to be honest) and the other being the Holo-Lawrence relationship (which is cute). Tomoko has no interest in economics and didn't really follow the storylines, but she watched the 2008 season without thinking it anything special. It's as if the show's deliberately avoiding excitement. Potentially action-packed scenarios (like ep.5) twist themselves around so that our heroes have nothing to do and the emphasis is all on the stuff that isn't action.
The show really is all about what it's like to be a merchant. This can be a bit cold-blooded, so for instance no one seems to have a problem with cheating, lying and betraying your business associates. When it happens to Lawrence, he just accepts it as a learning experience and congratulates the son-of-a-bitch who got the better of him. It's his own fault for not being clever enough to see through it. (He tries to be fair in his dealings, though, because a merchant lives or dies by his reputation.) Merchants are basically all about money and there's something almost off-putting about Lawrence's tendency to weigh everything in terms of profit and loss.
Similarly, corrupt city governments and the slave trade are accepted without question. That's just the way of the world. Besides, such issues can seem insignificant compared with the power of the church and the danger of armed revolution.
However there's also a human side to the series, which is the Holo-Lawrence relationship. It's a slow, open-ended romance. (The original novels stopped in 2011 after 17 volumes, of which this anime adapted the first six. The anime stops without really resolving anything, although it's clear how much both characters have grown and there's not much doubt about where they're going.) This is done very well. Their banter is charming. Holo is very intelligent, has superhumanly keen perception and rather enjoys deconstructing human male psychology and what they really like in women. However she's also ancient and frightened of loneliness. Meanwhile Lawrence is guarded, bad at commitment and has maintained an equanimical, smooth-talking shell for business purposes for so long that for all we know by now it's his true nature.
In one story arc, Lawrence's inability to respond properly to Holo actually becomes a plot point. They both grow. They're both capable of trying a bit too hard to do what they think the other one wants. Holo's easier to read than Lawrence and can also be flirty, to the point of teasing.
To be honest, ep.10 was the first episode that struck me as strong. Until then, it had been sort of interesting without being gripping. Eps.10-11, though, had the characters being harsh to each other, losing their cool and getting in real danger. (They're in a fiercely religious town in that story arc, while bankruptcy takes on greater weight when you realise that it would mean being sold into slavery for the rest of your short life. Oh, and you'd also have your head shaved and your teeth pulled out.)
Tomoko didn't watch Season 2, but I enjoyed it more than Season 1. The Holo-Lawrence relationship has moved on and the writers can do stronger things with it. It's good. I don't know if I'd recommend it to anyone, but it's certainly worth a look if the concept sounds intriguing.
There's another medieval economics anime, out there, by the way, which similarly even has its two leads being a human man played by Jun Fukuyama and a supernatural woman played by Ami Koshimizu. That's Maoyu (i.e. Maoyuu Maou Yuusha), but I think that's the more accessible to general viewers. Maoyu's scale is bigger. Its Demon Queen is trying to drag an entire world out of the Middle Ages, which involves outwitting her fellow demons and dealing with human Heroes, Knights, etc. who are technically her mortal enemies. This show, on the other hand, really is just about making a living buying and selling things.
Maoyu's also more fanservicey, mind you, since its Demon Queen is very buxom. This show has very little of that, despite Holo's lack of concern about nudity... but that's not what you'd assume from the show's American marketing. Not knowing how to sell the show, unsurprisingly, they used lots of pictures of Holo naked and gave everyone the wrong impression. (We never see nipples, by the way.)
It's a modest show, but quite good. I think you have to meet it halfway, in that it's making absolutely no attempt to be anything but what it is. It thinks medieval economics is fascinating. It expects you to agree, or at least to go along with it for the time being. Fortunately its storylines are indeed quite interesting if you're not put off by that kind of thing, while it's always nice to spend time with Holo and Lawrence.