Kantarou Ichinomiya is a writer (he specializes in folklore) and is a part-time exorcist. He has the ability to see youkai (like Japanese fairies and mythical creatures), and he has a powerful spiritual side and a really big heart. Usually exorcists exorcise spirits and demons and the like, but Kantarou reasons with them. When he was a small boy his powers scared people, so he was often alone. He spent most of his time with the youkai and became fond of them. He learned from them the power of names, and that if he names a youkai then it will obey him.
Kantarou has spent his adult life writing (his publisher is a frequent and annoying character), and searching for the "mythical" Oni-Eating Tengu (most bad-ass demon around, because it EATS other demons). In episode one Kantarou finds and names the Oni-Eating Tengu, Haruka. Haruka has lost both his memories and his Oni-Eating powers. The two form a friendship beyond master and servant and work together to help Haruka piece together his past.
I have learned a lot about Japanese mythology from Tactics, and although some stand-alone episodes do seem very "monster of the week" the overall plot and development of characters (even characters who seem minor at first get their own development) is at the center of the anime and manga series. The relationships between the characters are interesting as well; not quite love, not quite family, not quite mere friendship. There are a lot of exciting moments, and some very sad and tender moments as well, all revolving around Japanese culture (both mythological and contemporanious to the Taishō period of 1912-1926). The Taishō time period is pretty significant to the story, as it is historically a time of change from the traditiona into a more modern era, and signified Japan's opening to democracy and international influences. Traditional Japanese culture was at a sort of turning point, and that feeling exhibits itself in the anime.
There are only 25 episodes of Tactics in total, and it seems perfect. Unlike a lot of American cartoons that don't end, or are so episodic that they don't have a plot, Tactics has a clear story arc and a sense of completeness to it. It's a tight series, with some impressive interconnected and overlapping shorter segments. The cumulative effect and novel-in-stories feel of the anime makes it my choice for Anime with Best Plot Idea.
And Haruka is totally dreamy: