"GANTZ:O" is a 3D CGI anime film based on the manga series "GANTZ" by Hiroya Oku. The story centers on a group of individuals who die unexpectedly and are subsequently transported to a mysterious room dominated by a large black sphere known as GANTZ. The sphere assigns them missions to hunt and eliminate alien creatures, offering them a chance to survive and potentially return to their normal lives if they succeed.

The protagonist, Masaru Kato, is a high school student who dies while attempting to save a man from a subway accident. Upon awakening in the GANTZ room, he encounters other recently deceased individuals, including his childhood friend, Joichiro Nishi, and a young boy named Yoshikazu Suzuki. The group is equipped with advanced weapons and suits provided by GANTZ, which enhance their physical abilities and provide protection during their missions.

The primary mission in the film involves battling a horde of alien creatures in Osaka. The aliens are highly dangerous, possessing immense strength and unique abilities, making the mission perilous for the inexperienced team. As the group struggles to survive, tensions rise, and alliances are tested. Kato emerges as a natural leader, driven by a sense of responsibility to protect others, particularly Suzuki, whom he views as a younger brother.

Throughout the mission, the characters confront their own mortality and the moral ambiguity of their situation. The GANTZ sphere remains enigmatic, offering no clear explanation for its existence or the purpose of the missions. The film explores themes of survival, sacrifice, and the human condition under extreme duress, as the characters grapple with their fears and desires while fighting for their lives.

The narrative is fast-paced, with intense action sequences and a focus on the dynamics between the characters. Kato's leadership and compassion contrast with the more self-serving attitudes of some team members, highlighting the diverse responses to the dire circumstances. The film concludes with a mix of triumph and tragedy, leaving the ultimate fate of the characters and the nature of GANTZ unresolved, consistent with the source material's tone.