Kurosaki Ichigo is a teenager gifted with the ability to see spirits. His life is drastically changed by the sudden appearance of a Shinigami (literally, Death god) - one who governs the flow of souls between the human world and the afterlife - named Kuchiki Rukia, who arrives in search of a Hollow, a dangerous lost soul. When Rukia is severely wounded while trying to defeat the Hollow, she attempts to transfer half of her Reiatsu (literally, Spiritual pressure) energy to Ichigo so that he can defeat the Hollow. However, Ichigo takes almost all of her energy, transforming into a Shinigami and allowing him to defeat the Hollow with ease. With her powers diminished, Rukia is left stranded in the human world until she can recover her strength. In the meantime, Ichigo must take over Rukia's role as a Shinigami, battling Hollows and guiding souls to the afterlife realm known as the Soul Society.

[Source: Wikipedia via AniDB]