Future Diary is chapter 1 of the Future Diary manga.
Yukiteru Amano wakes up on what seems to be a normal morning, checking the date on his cellphone. To his confusion, there are several events scheduled to occur later in the day. Brushing off his puzzlement, Yukiteru begins his daily routine by tossing a hand dart at one of the dartboards on the wall of his room. The dart hits a bulls-eye, fulfilling the first prediction on his phone.
Later, Yukiteru watches a television and learns about a serial murderer in his area; to his astonishment, his phone has predicted this beforehand. Convinced that it's just a coincidence, he heads to school, finding several more of the phone's predictions coming true.
Afterwards, Yukiteru encounters his imaginary friend: Deus Ex Machina, the god of time and space. Deus informs him that his phone has become a diary that tells the future for the next ninety days; when a day passes, the diary's entries will be replaced by the next day's future. Deus also warns Yukiteru that if the phone is destroyed, so is the holder's future, meaning that they will die.
Yukiteru later utilizes his phone in order to get top marks in his class, which irritates other students. He uses his phone in order to evade their upcoming ambush, becoming delighted with the use and efficiency of his new power. He then notices Yuno Gasai looking at him, but brushes it off as a coincidence; special powers or not, there's no way the school idol would be interested in him. However, he then realizes that Yuno has made a clay model of another of his "imaginary friends": Deus Ex Machina's assistant, Muru Muru.
Suddenly, Yukiteru notices a "Dead End" entry in his diary, warning that he will be chased and murdered by the serial killer he heard about in the news. As he begins to panic, Yuno appears behind him, confirming that the phone's prediction is true. A frightened Yukiteru flees from the classroom, noticing that his immediate future has changed on the diary. He then enters an elevator, only for Yuno to jump in after him, having somehow anticipated that he would come here.
Realizing that Yuno must also carry a Future Diary, Yukiteru begins to fear for his life. He considers stabbing her with a dart that he has concealed in his bag, but Yuno abruptly kisses him - afterwards, she notes that he did not stab her, declaring that as the future. She then points out the serial killer walking around outside, recognizing him as the third Future Diary holder, and explains that she knew the serial killer was pursuing Yuki because of her own diary. Yuno shows Yukiteru her phone, calling it the "Yukiteru Diary"; it shows her his future in ten-minute intervals.
As Yukiteru realizes that Yuno is an extreme stalker obsessed with him, Yuno changes the elevator's destination from the 14th floor to the 17th; where Yukiteru is destined to be killed. She tells Yukiteru that there are several diary holders, all participating in a "survival game"; they must eliminate each other until only one remains. When the elevator arrives at the rooftop, Yuno announces that running from the serial killer is pointless; instead, they are going to strike first. She discloses her plan to Yukiteru, telling him that it involves his dart.
Some time later, the Third reaches the rooftop as well. As he walks out of the elevator, Yukiteru frets from his hiding place; he has only one dart, and thus only one chance to complete Yuno's plan. He recalls Deus's advice that the Future Diaries are the futures of their holders; if he manages to destroy the Third's diary, the latter will die. Accordingly, Yukiteru reveals himself and tosses his dart, which penetrates the Third's phone with lethal precision. The Third barely has time to process the attack before his body collapses into a vortex that vanishes from existence; disappointed by how simple the death was, Yuno suggests that they have dinner at Yukiteru's house, as his mother won't be back tonight.
Later, Yukiteru confronts Deus, who tells him that he never mentioned that Yukiteru alone was a diary holder. He reveals that he isn't just Yukiteru's imaginary friend; he really is the God of time and space, whose life is coming to an end. The Survival Game is being held to find a successor for Deus; whoever outlasts the others will become the new God. As Yukiteru tries to process these revelations, Yuno appears behind him and vows to protect him; naturally, that just makes him even more apprehensive.