Kurono Kei and his ex-elementary school classmate, Kato Masaru have survived the first two ordeals that the unknown black sphere Gantz has sent them through. Exploding body parts, struggling to stay alive till the last seconds and seeing your fellow comrades fall in a pile of blood and gore are norm to them now. They are aware now that Gantz can call them up along with any new deeds, at any time for another confrontation with aliens. 
Will Kato's experiences in the Gantz world give him the same courage in the real world? With fellow veteran Gantzer Kei Kishimoto currently staying at Kurono's home as his "adopted pet", can Kurono stave off his growing lust for her mammaries? 
What the heck is Gantz? 
(Source: anime-source.com)