In Germany, Hans Beckert is an unknown killer of girls. He whistles Edvard Grieg's 'In The Hall of the Mountain King', from the 'Peer Gynt' Suite I Op. 46 while attracting the little girls for death. The police force pressed by the Minister give its best effort trying unsuccessfully to arrest the serial killer. The organized crime has great losses due to the intense search and siege of the police and decides to chase the murderer, with the support of the beggars association. They catch Hans and briefly judge him.
There have been a rash of child abductions and murders in Berlin. The murderer lures the children into his confidence by candy and other such child friendly items. Everyone is on edge because the murderer has not been caught. The most substantial pieces of evidence the police have are hand written letters by the murderer which he sent to the newspaper for publication. Unknown even to himself, a blind beggar, who sold the murderer a balloon for one of the child victims, may have key information as to the murderer's identity. The murder squad's work is made even more difficult with the large number of tips they receive from the paranoid public, who are quick to accuse anyone of suspicious activity solely for their own piece of mind that someone - anyone - is apprehended for the heinous crimes. Conversely, many want to take the case into their own hands, including the town's leading criminals since the increased police presence has placed a strain on their ability to conduct criminal activity. Although they both have the same end goal of capturing the murderer, the police and the criminals seem to be working at cross purposes, which may provide an edge to the murderer in getting away.
In Berlin in the early 1930's, children are being lured to their death by a psychopathic killer. In the space of a year, 8 children have been murdered. The police have redoubled their efforts to find the guilty man but have yet to find him and citizens are beginning to dispense their own justice on otherwise innocent people. The heads of the city's criminal element are paying a high price due to the increased police presence and decide to find the psychopath on their own. They approach the beggars union to have their members blanket the city with spies. They're successful in finding the killer and put him on trial in their own special court but the police make progress and have their own views on how justice should be administered.
Someone is murdering children in a German town. The police are doing all they can to solve the case but, after several months, several murders and exhausting work, still have no clues. Their methods of trying to find the murderer start to adversely affect the local criminal community. Due to this, the local organised crime syndicate takes it upon themselves to find the murderer and mete out punishment.
A psychotic child murderer stalks a city, and despite an exhaustive investigation fueled by public hysteria and outcry, the police have been unable to find him. But the police crackdown does have one side-affect, it makes it nearly impossible for the organized criminal underground to operate. So they decide that the only way to get the police off their backs is to catch the murderer themselves. Besides, he is giving them a bad name.
When the police in a German city are unable to catch a child-murderer, other criminals join in the manhunt.
- It's noon. Concerned parents are lined up outside the schools waiting to escort their kids home for lunch. Signs along the street ask "Wer ist der Morder?" (Who is the murderer?) of eight children killed over the past year. Little Elsie Beckmann [Inge Landgut] has just become victim number nine.
Who is the murderer? It could be anyone (the viewer knows that it is Hans Beckert [Peter Lorre]). Neighbors are turning in neighbors and fingering strangers in the street just for talking to a child. The police are on the job 24 hours a day, but all they have turned up so far is some Ariston cigarette butts, some sugar grains, and a bag that held candy. They have investigated over 1,500 clues and have compiled 60 volumes of documents. Every thicket, every bush, every hole in the area has been combed. Every place from flop houses to underworld hangouts are being raided every night.
In fact, the police investigation has been so thorough that the Underground is getting worried about the shakedowns. Underworld boss Schränker [Gustaf Gründgens] has come to the conclusion that the child killer must be found before the police ruin business. After a lengthy discussion with all the Underworld bosses in attendance [there is a wonderful scenario where the Underworld bosses discuss strategies while the police do the same, such that it hard to tell the difference between Gangsters and Police], they decide that the best solution is to monitor all children at all times and that the best persons to do that, the only ones who can be seen on the streets without arousing suspicions, are beggars. So, every street has its beggar sitting in a doorway, having a smoke on the corner, watching the children and noting anyone following or with a child.
A break comes in the person of a blind balloon peddlar [Georg John] who hears a man whistling a tune and remembers the same tune being whistled by a man who purchased a balloon on the day Elsie Beckmann was murdered. He alerts his block's beggar, who follows the Beckert, who is leading a little girl into a candy store. When Beckert makes the mistake of throwing an orange peel on the sidewalk, the beggar pretends to slip on it and, while clutching Beckert for support, he transfers onto the back of Beckert's shoulder a big "M" that he previously chalked on his palm. The beggars are now able to follow the man with the "M" on his coat.
Meanwhile, the police have been attempting to trace the origin of a postcard sent by the child killer to the local newspaper. While investigating released criminals and mental patients that fit the profile of the killer (lazy, indolent, and of strong and pathological sexuality), the police come to Hans Beckert's apartment. Beckert isn't home, but the landlady shows them in. Searching Beckert's room, they find some Ariston cigarettes and a red pencil, the likes of which was used to write the postcard. Certain that Beckert is the killer, the police lay in wait for Beckert to return.
But Beckert is on the make. The little girl that he is with suddenly notices the "M" on his shoulder and offers to wipe it off. Certain that he has been caught, Beckert runs into an office building. The beggars contact Schränker who sends a group of gangsters to comb the building and capture the child killer. Unfortunately, the night watchman sounds an alarm, alerting the police. In the minutes before the police arrive, the gangsters find Beckert, and everyone, except for burglar Franz [Friedrich Gnass], manages to get out before the police arrive. To make Franz sing, Detective Lohmann lies to him that the night watchman at the building was killed during the gangsters' raid. The strategy has the desired effect. Fearing that he is going to face a murder rap, Franz tells how the gangsters have captured the child killer and taken him to an abandoned distillery to stand trial.
Meanwhile, in the basement of the abandoned distillery a kangaroo court is in session, comprising of the leaders and the members of the various criminal organizations. When Beckert explains to Schränker that he is driven by a voice and an evil impulse he can't control, it is voted to eliminate him so that there is no chance of him ever getting free to murder another child. Only Beckert's appointed defense counsel [Rudolf Blumner] argues that, since Beckert is driven by an uncontrollable impulse, he cannot be held responsible for his actions and should be turned over to the police so that the state can render him harmless. At that moment, the police enter the distillery and escort Beckert away.
Epilogue: As Beckert's real trial progresses, the mothers of the slain children listen to the verdict. The ultimate and final comment comes from Frau Beckmann [Ellen Widmann] who says that none of this will bring back the children and that mothers must keep a closer watch over them. [Original synopsis by bj_kuehl]