The main character is named Yuki, a very solitary boy who keeps an observational journal on his phone. He has no friends, by choice, and is in every way an observer of his own life, not actually taking part emotionally. His only 'friends' are imaginary beings, a god named Deus, and a little sprite my creature. One night while talking with his imaginary friends, Deus tells him that things are going to change, and the next day, he finds that his diary posts on his phone have already been written for the day, telling him what will happen. He happily takes advantage of his soothsaying device, until on day, a girl in his class approaches him, and reveals that her phone is also special, also able to predict the future. This freaks him out obviously, but it isn't until that night when he meets Deus, as well as several other faceless strangers, that he finds out what's going on. He, as well as several other people, have been granted devices that tell them the future, and they must use these devices to find and kill each other. When a person has been targeted by another diary user, their own diary will tell them that they have a 'dead end', and they must try to change their fate if they can.
The last person standing becomes god. And all diaries are different, and reflect the personality of the user. Yuki is an observer, so his diary predicts events that will happen around him, but there are other types of diaries too. Yuki then is faced with a battle royale to inherit the seat of god.
I had some annoyances while watching this. Yuki is a very timid boy, which is fine, but he doesn't show any significant growth until very late in the show. He's always whining and crying, and the other characters comment on this as well throughout the story. What was refreshing is that this personality is usually given to females in anime, but all of the females showed various levels of considerable strength, which was nice. Yuno's character, the girl who is apparently obsessed with Yuki, was also troubling at times, but her character became more of a mystery as the show went on, so I was more curious about what her story was than annoyed by her rabbid obsession.
This is definitely not a pg show. There is physical, sexual, and emotional violence, as well as some nudity. It is a pretty good show however. The plot becomes increasingly complex and surprising, and will leave you guessing to the very end, after the credits. It's only 26 episodes long, and that goes by pretty quickly. It's one of those shows that you'll want to marathon. And while you might be annoyed by some of the characters at various points, you won't be bored.