Shiyouji Shunpei is alone in a hotel room when three strange little creatures come in through his wall. They're looking for some human to make a contract with, and they're not really picky about who that human might be. Before he knows it, Shiyouji is contracted with the three annoying little guys, and it seems like he's supposed to work with them to defeat other creatures that kill humans. He meets with another woman in this line of work, who insists he quit his day job to focus on his new responsibilities. Shiyouji is not an especially great guy, but it looks like he's going to have to be some kind of hero.

(Source: MangaHelpers)