The story is told from the perspective of Keiji Kiriya, a new recruit in the United Defense Force. Despite equipping its soldiers with powered exoskeletons, the UDF is losing its fight against the mysterious "Mimics", extraterrestrials which have laid siege to Earth. Keiji is seemingly killed on his first sortie after killing an unusual-looking Mimic, but through some unexplicable phenomenon, he wakes up and finds that he has returned to the day before the battle. As this process continues, he finds himself caught in a time loop as his death and resurrection repeat time and time again. Keiji's skill as a soldier grows as he passes through each time loop in a desperate attempt to change his fate.
After several dozen loops, he realizes his fate is similar to that of Rita Vrataski, a prominent ace who preferred to use a battle axe rather than a firearm. He uses his knowledge of the day to get close to her and her mechanic, from whom he gets a copy of her massive axe, a weapon he learns to use well; the boltgun that most troops are issued quickly runs out of ammo and jams easily. Realizing that he is a fellow looper, Rita confides in Keiji, telling him of the system the Mimics use: on death, they have the ability to send a signal into the past, allowing them to see the future and change their behaviour to avoid that fate. In each group of Mimics, there is a central nexus that can cause the day to loop, as well as several antennae, all of which signal the loop to reset; Keiji became trapped in the loop as a result of contact with one such antenna. To escape as Rita once did, Keiji must first kill all the antennae and then the nexus. The Mimics constantly adapt to Keiji's attacks, but he and Rita manage to eliminate the nexus, only to have the loop reset again with Rita forgetting what has transpired. After telling Rita this, she acknowledges that they missed one antenna. On the 160th loop, they proceed to eliminate the antennae again. However, Rita then attacks Keiji once they are out of sight from allied forces, explaining her hypothesis that being trapped in the loop has modified their brains. In essence, both of them are similar to the antennae, meaning one of them has to die before killing the nexus; otherwise, the loop will continue indefinitely. Reluctantly, the two do battle and Keiji manages to mortally wound Rita and stays by her side as she dies. Before Rita dies, Keiji confesses his developing feelings for her before she tells him to win the war quickly and prevent anyone else from suffering the same fate they just did. Afterwards he slaughters the remaining Mimics and destroys the nexus. Weeks later, he is hailed as a new hero of the United Defense Forces while he silently reflects on what transpired and the sacrifice needed for them to win the battle. Afterwards, he decides to paint his exoskeleton blue in honor of her memory.
All You Need Is Kill was nominated for the "Best U.S. Edition of International Material—Asia" category at the 2015 Eisner Awards.