Massive black clouds emerge from nowhere all over the world, hiding unknown ships, that begin to attack all major cities. The largest most powerful military in the world, the American military, is obliterated, in mere hours.
Tokyo is shown being attacked by various types of aliens, both as humanoid or animal-insect like in appearance, all with black armor on them, with similar designs to the gantz suit.
Some of these aliens have wrist watches or other items that show pictures of the Gantz hunters, which they are suppose to be killing. One alien in a flying vehicle, removes his helmet to show they look somewhat human, only with an extra eye to the side, and are much larger than humans.
Some animal like aliens herd humans into a large cage, that then lifts up on legs, part to a massive structure, that appears to be the mothership of the aliens. Many humans are shown lying about, dead, or injured, having pieces of them cut off by the aliens, around the streets of Tokyo.
The people that are herded in the cages are taken to a place where they are sprayed down with some dissolving agent that dissolves away their clothes and possibly kills any bacteria from them. They are then taken and hung upside down with their blood getting drained out, and then taken where they become food for the other aliens. The few of them that escape this process and reach the surface, are being hunted for sport, adopted as pets, or simply killed like bugs.
During the beginning of Katastrophe and the invasion, in a monitor of Seiichi Kikuchi's laptop is seen that the time for the event has come and an encryption code appears on screen as 666.
A reference to the apocalypse.
Katastrophe is the German word for Catastrophe. In the original Japanese version of Gantz, the German word is used.