From the beginning of the series, one will never deduce that the anime revolves around a child trying to save the day as the main character is already a full fledged adult. Our protagonist is Satoru Fujinuma, a 29 year old pizza delivery man and a unfortunate manga artist struggling to live out his somewhat normal life. Underneath the layers of his normalcy lies a power the protagonist has no control over . The ability to go back minutes in time before something bad happens, an ability that has helped him prevent countless tragedy which he dubbed revival.
After years apart, Satoru's mom comes visiting to spend some time with her only son. A flashback reveals that when Satoru was just 10, a tragedy happened to one of his classmates that was kidnapped and killed with a corpse discovered months later. He blames himself for it as he saw her alone in the park the very night she was killed and wished he talked her into going home then . Yuki young adult who stutters was arrested for the crimes as all evidence points to him being guilty all to the dissatisfaction of Satoru but he had no prove to back up his claims.
In the present Satoru and his mom goes shopping and revival rewinds time again but he is unable to pinpoint the anomaly so he tells his mother to look around for anything strange . She spots a man trying to abduct a child and when he notices that she was catching on he left the child in quite the hurry. Certain that she had seen the face before, she goes back to her son's apartment to brainstorm. While she's there a stranger comes in the house stabs her and vanishes.
Satoru comes back home to meet his mother dead and bleeding on the floor, scared of being seen as the perpetrator, he runs away but finds himself in another revival but this time it wasn't minutes but over 18 years back in time. Feeling like he has been given a second chance, our hero vows to save his mother from her tragic fate as he still mains the main of a 29 year in a child's body. He also encounters Kayo Hinazuki, the classmate that was murdered when he was a child further deducing that his mother's murder and this one was connected he decides to befriend her to ensure that he achieves his desired future.
10 year old Satoru tries to get close to Kayo and soon discovers that she is abused emotionally and physically by her own mother which was the reason he lingers around the streets before returning home late ultimately causing her abduction and murder. After a tough struggle, he eventually befriends her and even celebrates his birthday today with her as they both share the same birthdate
The main character still in child form continues to investigate the entire event that led up to the abductions and killings even getting help from his childhood friends to unravel this mystery. During his search, Satoru continues to make discoveries that was leading him to the culprit but before he could conclude it all, an event happens that sends him into a comma till he wakes up at his real timeline.
He wakes up to find out that his mother, Kayo are both alive and she(Kayo ) was married with a kid meaning that his primary objective was achieved. The real culprit comes back to the hospital to finish him off and it is revealed to be his elementary school teacher he disguises himself as a nice person just to abduct and murder kids ( a sociopath). He dies while struggling with Satoru at the top of the hospital floor finally concluding the entire case.
The anime starts off with a mature protagonist but most of the plot revolved him in the body of a 10year old so it definitely qualifies as an anime with a child protagonist.
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PS: pictures are screenshots from IMDB