Detection and Subsequent Action is the first chapter of the manga Ajin: Demi-Human, written by Tsuina Miura and Illustrated by Gamon Sakurai.
We are first introduced to Kei Nagai, a teenage boy who is talking to his friends about studying throughout summer break for medical school. Soon after he spots Kaito, referring to him as Kai in his head, hinting that they have/had some sort of relationship, but brushes him off as a "freak" as soon as his friends question whether or not Kei knows him or not, one of which goes as far as to call him a demi-human. They continue walking for a bit, tlalking about demi-humans, questioning what exactly they are.
The manga then cuts to a classroom that Kei is in, and the professor states that the class will be talking about demi-humans that day. He asks what the class knows about them first, and one student answers by saying "They're immortal. They can never die". Kei seems to be uncaring about the subject, brushing it off by saying "Demi-humans... I can ignore this class". The professor continues to say that they also emit a so-called "unique voice", not clarifying what that means, before saying that this is all the public knows.
He says that they were first discovered in Africa 17 years ago, when what the locals called the "Soldier of God" appeared, which hints that he was a soldier that was killed in the battlefield before being resurrected. These rumors were then confirmed by the US army after failed attempts to cover it up. The professor said that 46 demi-humans have been discovered, 2 of which are in Japan, and that many more are likely to exist, since it's impossible to know until you die. One student says that there is a so-called "giant reward" for finding a demi-human. When the professor refers to a scenario like that of gaining ownership of "captured prey", Kei asks whether or not demi-humans are people or not, to which the professor denies, saying that they aren't.
The story then cuts to a flashback containing Kei, his mother, Eriko (Kei's sister) and a deceased puppy. After Kei asks if they're not going to the hospital, his mother says that the pet shop gave them a "defected one", and that she should sue. The girl asks Kei why the puppy had to die, and Kei responds by saying that's just what happens with sick things. She asks if people just die if they are "defected". Kei attempts to comfort her by saying of saying that it isn't true. Then Kei asked why people die and it's not like they need to. Then, a tall, almost inhuman silhouette appears, crudely repeating Kei's words, before the flashback ends.
Kei, now back in reality continues to walk, unaware of his surroundings, thinking to himself about his flashback before getting ran over and subsequently killed by a speeding truck. The driver, now panicking, tells everybody to look at the traffic light which was clearly red, taking a picture of it, before questioning whether or not to take a picture of the body. Kei is then resurrected from the dead, his body healing itself of all its wounds, revealing Kei to be a demi-human. An utterly shocked Kei asks everybody to please hear him out, his friends saying that of course they believe him, however, Kei knowing that there is a large bounty and fame for finding a demi-human. He sees through their lies, and screams, paralyzing everybody present.
Kaito, who was walking by a few police officers, discovers that Kei is a demi-human from a loud-mouthed housewife and rushed after him, clearly worried.
Kei is then seen in some sort of forest talking to himself in panic after finding out that he is an immortal demi-human, terrified by the idea of being tortured and researched on with no hope of death. Remembering that the one person he can rely on is Kaito, he calls him, asking for his help. Hesitant at first to tell Kai where he is, Kaito then assures Kei that he can trust him. Kei does so, and Kaito rushes to his aid. Before Kai can reach him however, an officer finds him in the forest, saying that he'll "take care of you myself".
Kei tricks him however, and manages to use the opportunity to run away. While running away, Kei trips, and the officer catches up to him. While it seems like Kei will be captured, Kaito comes to his rescue, drop kicking the guard in the back. He does not knock him out however, and so while Kei greets Kaito, the officer alerts the rest of the police force of the demi-human's location.
Kei mocks the officer saying that he's catching him, but the black silhouette appears again, crudely copying Kei's words again.
The chapter transitions to July 23 19:47, with Kei and Kaito on a motorcycle. Kei apologises to Kaito for calling him a freak that morning before saying he isn't human, Kai saying that he doesn't care about any of that.
Finally, we transition to an unknown party that seem to be tracking Kei, different from the police force, hoping to secure him before another party captures him, saying "Kei Nagai... We're not letting you get away".