Gantz is an Anime which falls under the genres of action. It reaches up to 22 Episodes.
A typical high school student named "Kei Kuruno" goes back to his apartment by riding a train but this time he finds "Masura Kato", a friend from ages ago, who went off to save a drunken homeless person who fell on the tracks. He called for Kei's help, which ended up with the two being hit by the train.
The two died and were put in a room with many other dead people. There was a black Spheare in the room which soon opened up with a man inside named "Gantz." Gantz told the dead to elimanate a certain alien to be given a second chance and live again.