Kenji is a rather nervous high school mathematical genius, whose side job involves being a mod for a popular OS/Social Network on steroids called OZ.
Natsuki is an attractive fellow student at his school, who is going to attend a family gathering for the 90th birthday celebration of her great-grandmother, Sakae.
Given Natsuki's age, and her great-grandmother's Sakae’s rather traditional attitudes, the story begins when Natsuki feels a little compelled to put great grandma Sakae at ease concerning her future by bringing a boy home. So she decides to rope in Kenji into a little scheme in which he poses as her fiance. He agrees and goes with her to her family's ancestral residence out in the country. There, Kenji meets rest of Natsuki's large family or clan, and even gets to stay overnight (talk about lucky). However, that night, he receives a mysterious message on his cellphone of a mathematical puzzle to be solved. Proving the adage that humans are the greatest threat to electronic security, the message turned out to be a phishing scam, the puzzle turned out to be OZ's encryption key, and Kenji answered and solved it. This leads to an AI take over of OZ, and it is up to the Kenji and Natsuki's family to stop it and save the world.
Details
Don't let the graphics and transitions to virtual reality fool you. This film is often called "down-to-earth" for a reason. Its because this film is kind of about a wide-view of something closer to home (pun intended): The family; the good, the bad, and the ugly parts of it. The viewer is sort of given Kenji's perspective on that family, that of an outsider. This is unlike the intimate view of family that is present in "Wolf Children" (Watch it. Watch it now. Are you watching it yet?)
Let's face it, When it comes to much of Natsuki's family, you won't or can't remember half of their names or track their personalities. For the most part, they are a ton of lively faces, who are all blended together as people as far as you can see. They aren't uninteresting, just hard to get a feel for as individuals. What you can see is that they care for each other, and interact lovingly with each other, and eat together.
The bad is that some of them are annoying. Their petty jealousies can make situations a difficult, the young ones can be utter brats, and their misplaced priorities can derail even the best constructed plans. Sometimes, they even severely embarrass you right in front of guests.
The ugly is that sometimes families bear unpleasantness in their collective history. Sometimes, things like infidelity and mistreatment occur, and hostilities can arise between what were once close relatives, and even between parent and child, over the actions taken by one of them. If things get ugly enough, consider the "family contract" irredeemably broken.
But the good is that family can reliably be there when things get difficult. Where one member can't cut it, the others can share the struggle. They are connections that can pull in for the best when the pressure is high. And even at their worst, if each member gets their priorities straight and are willing to actually change for the better, and do the right thing when the time calls for it, there is a hope that they can recover.
TL;DR: Evil AI threatens world and Family. A good Family rallies and strikes back.