"Hello World" is a science fiction anime film set in a futuristic Kyoto in the year 2027, where advanced technology has transformed society. The story revolves around Naomi Katagaki, a reserved and bookish high school student who leads an uneventful life. One day, he encounters a mysterious man who claims to be his future self, having traveled back in time from the year 2037. This older Naomi explains that he has returned to alter a tragic event involving Naomi's classmate, Ruri Ichigyō, a girl he secretly admires.

The older Naomi reveals that Ruri will soon suffer a life-threatening accident, leaving her in a coma. To prevent this, he tasks the younger Naomi with protecting her and changing the course of their shared future. As the younger Naomi grows closer to Ruri, he begins to uncover the complexities of their relationship and the true nature of the timeline. The narrative delves into themes of love, sacrifice, and the consequences of altering the past, as the two Naomis navigate a web of secrets and technological anomalies.

The story takes a dramatic turn when it is revealed that the older Naomi's timeline is not as it seems. The futuristic Kyoto is actually a simulated reality created by a quantum computer, and the older Naomi is a digital consciousness attempting to rewrite the past to save Ruri. The younger Naomi must confront the ethical and emotional implications of his actions, as well as the possibility that his reality may not be as genuine as he believes.

The film explores the interplay between human emotions and artificial intelligence, raising questions about the nature of existence and the boundaries of technology. The characters are driven by their desires to protect loved ones, even at the cost of their own identities. The narrative culminates in a poignant resolution that intertwines the fates of Naomi, Ruri, and the simulated world, leaving viewers to ponder the fragility of reality and the power of human connection.