A killer is roaming the streets of Italy, meticulously searching for a particular victim, setting a dark and unsettling tone for the story. The killer breaches the victim’s home in a chilling invasion of privacy and ultimately murders her. Before leaving, he catches a fleeting glimpse of her daughter, Ines, in a photograph, hinting at a sinister connection. The murderer then disguises himself as an escort and later kills Kathy Adams, in a desolate field, further cementing his reputation as a relentless and elusive criminal.
Meanwhile, Giulia Torresi, a young woman preparing for her wedding to Mario Gerosa, a wealthy clothing designer, receives a mysterious phone call while choosing her bridal dress. The caller is cryptic and hangs up whenever she asks for their identity, adding an element of intrigue and suspense. That very night, another unsettling call is made to Kathy, who is attending a party. The caller’s voice is disturbingly familiar and is revealed to be someone close to the gathering. This individual follows Kathy home, leading to her tragic murder, heightening the sense of danger surrounding those on the list.
In a shocking twist, Giulia is attacked on a train during her honeymoon, and it appears she has been killed. However, her funeral is staged as a ruse to trap the actual killer, revealing her survival. She awakens with no memory of her attacker, which complicates the investigation. The police suspect her new husband Mario, but Giulia recognizes a former hotel employee as one of the victims, based on her memory. She mentions a lover of this employee, who was accused of theft, and Rau, the suspect, is brutally interrogated but maintains his innocence, lacking a solid alibi.
Confused and unsure of herself, Giulia confesses to Mario that she doubts her suspect. Noticing that Mario consistently has a half moon plaque left at each murder scene, she recalls a memory of a frequent hotel guest with a similar mark on their key chain. She and Mario visit the hotel where she worked, but find nothing definitive. Realizing that the hotel’s records might have been tampered with, they discover that a page from the guest register—dating from the time of the murders—has been ripped out. When they find the missing page, it lists victims including Anita Ferri, Kathy Adams, Elena Marchi, Concetta de Rosa, Anna Sartori, and other women, including herself and Ines.
Unable to relax, Mario and Giulia continue their investigation by visiting Elena Marchi, who now resides at a mental institution. Unfortunately, they arrive too late—the Elena they find is deceased. Mario chases a suspicious figure into a dark corridor and is slashed on the shoulder, while Giulia glimpses the killer escaping, though she cannot describe him. This leaves her with a haunting sense of frustration, and suspicion begins to turn toward Mario himself.
The police’s focus shifts to Concetta de Rosa, another victim on the list. Vismara, an inspector, warns her that she might be next. Mario, pressed by the growing mystery surrounding the American man suspected of being the killer, consults a sketch artist. The resulting drawing is shared with a local parish priest, who recognizes the man as a friend of someone named Barrett. This leads Mario to Barrett, who confirms the sketch as a man called Frank Saunders. Barrett explains that Frank was interested in a girl and as a result, they haven’t seen him in some time. Mario begins to suspect that one of the women on the list might be connected to Frank.
Further investigation reveals that Concetta de Rosa had not been attending her usual church services, prompting suspicion. When police check her residence, they discover she has been murdered—shot in a confession booth. The killer gains the moniker "The Half Moon Maniac" because of a recurring symbol linked to him. Clues lead Mario to an address associated with Frank Saunders, but he learns Frank died in a car accident two years earlier. Anna Sartori, another woman on the list, arrives in Italy but is immediately scrutinized by the police, as her presence suggests she might be another target. Her husband explains her trauma during her time abroad, adding layers to her troubled history.
As Mario investigates further, he visits the hospital where Frank died and learns that Frank might have been murdered by someone behind the wheel during the crash. The doctor suggests the involvement of a woman, possibly connected to Frank’s past. In a dramatic turn, Mario storms Barrett’s house, only to find him drugged. Returning to the parish house, Mario tries to uncover the identity of the officiant at Frank’s funeral but is met with silence.
Anna, unable to secure her safety, sends her identical twin Maria out to retrieve a newspaper, but Maria is mistaken for Anna and is murdered. Frustrated and desperate, Giulia devises a risky plan to publicly announce her状态，试图引诱嫌疑人暴露自己。不幸的是，精神压力让她晕倒，失去了追踪线索。此时，警方开始怀疑Barrett，并前往他的住所，但他已自杀。Mario怀疑这是否真的是自杀，进一步加深了谜团。
随着线索的揭露，Mario拜访了Anna，从她口中得知她当年在一次车祸中其实是司机，她离开Frank是为了保护其丈夫。她揭示，她和Frank的兄弟之间有着复杂的关系，正是这段关系导致了悲剧的发生。Mario意识到，Frank的兄弟正是他曾经见过的人。在追查过程中，Giulia在家中听到唱片突然停止，发现唱片机上出现了半月形的标记，暗示着凶手可能在她身边。
最终，Giulia遇到扮作牧师的杀手。他坦言，所有的一切源于被害女性背叛了他的兄弟，导致他在一场车祸中丧生，并且他要为兄弟的死复仇。他还承认自己杀死了Barrett，伪装成自杀以隐藏真相。当他试图勒死Giulia时，Mario及时赶到，将杀手淹死在泳池中，结束了这场令人毛骨悚然的追捕。
整个故事充满了悬念与阴谋，逐步揭示出关于人性扭曲与复仇的深刻主题。[The Half Moon Maniac]的谜题逐渐解开，也展现了正义与黑暗交织的复杂关系。
