It centers around the travails and battles of Ushio Aotsuki, who is constantly being stalked and aided by a gigantic, supernatural and sometimes invisible tiger-like monster (somewhat like Calvin and Hobbes).

Ushio's family maintains a Shinto temple in Japan, where ~500 years ago, his samurai ancestor battled that same tiger to a standstill, and eventually trapped him against a rock using a blessed spear called the "Beast Spear", which in addition to its intrinsic formidability as a weapon, can also endow its wielder with the martial ability of the ancestor who originally wielded it. Ushio accidentally unsealed the cave Tora was trapped in.

Initially, Ushio refuses to remove the spear pinning Tora down (as Tora made the strategic blunder of mentioning that the first thing he would do upon being freed would be to devour Ushio), but Tora's unsealed presence summons various monsters who wish to fight him, and by extension Ushio. Ushio is forced to free Tora, who would like to eat Ushio, but dares not since Ushio keeps the spear close by. The series focuses on their relationship, interspersed with battles against mythological foes, and with Tora's attempts to grapple with modern life.

The story also includes Ushio's relationships with two girls: the tomboyish Asako Nakamura and the feminine Mayuko Inoue.

Episodic in nature, most of the manga is a series of fights with different youkai or other creatures from Japanese legend. But at thirty-three volumes, the story continues far past the events of the anime, and leads to an epic conclusion.

(Source: Wikipedia)