Strange suicides have been taking place in New York. One day, four girls from the same high school wake up in the morning feeling tired and dizzy and not being able to remember anything about the previous night. In school, they find out that one of their classmates has committed suicide. School is canceled for the rest of the day, but instead of going home, the girls are drawn to a park by butterflies only they can see. Suddenly a man and a woman approach the girls, telling them that they all died the previous night.
Kate, Rose, Rachel, and Claire are four New York girls who cannot recall what events transpired the previous night. While at school news spread to the student board that the NYPD found Lize's body left at Central Park. Kate, who was close friends with Lise, breaks down and takes the rest of the day off. As the night falls, she and the other three girls are led by butterflys that only they can see to the same place and are unexpectedly approached by a woman wearing a black suit. She asks them if they can remember the previous night when all of them died.
The four girls are at the mercy of a man whose behavior sounds exactly like that of a rabid dog. The stranger is run over by a bus soon after Rachel displays a supernatural ability to break free from his grasp. Aghast and in shock all four are approached by the mysterious woman, who calls herself Lula, and explains them that are living on borrowed lives and from now on at every night they will be needed to hunt and kill demons for 'them'. If they break the contract, all will die.
Lize's father asks Kate about his daughter's diary and thanks her for being friends with Lize. The four girls become upset upon finding Lula also attending Lize's funeral. She gave them a red seed and explains to them to plant it if they want to know the truth. She later takes the four girls to witness another group of girls in the same situation as them, but having lost their will to live as they are brutally murdered by another beast man. The four girls plant the seed, and it blooms into a red flower at night, influencing the girls' memories as they finally recall the fateful night when they found Lise dead and were murdered by orders of a man in a white suit.
Kate starts to befriend the other girls to help them cope with their current situation, quickly earning Rose's trust and friendship. Both decide to pay a visit to Rachel and Claire. However, the socially withdrawn Rachel disdainfully chases the two of them out of her house alleging that it is no use to resume their normal lives since all are already dead. Meanwhile, Claire by herself is searching for the place where they all supposedly died.
Rachel breaks down due to having killed another beast man in another night mission forced upon them by Lula. Rose collapses in tears feeling herself unneeded when her siblings tease her that they don't want her. Kate and Claire go and investigate the manor where their deaths took place to find any clues about the truth.
When the girls are once again summoned for another fight Rachel loses her sanity and escapes. Kate and the others follow after her, learning from Lula that if even one of them strays from the group during a mission then all will die. Rachel is found and brought to her senses but has lost the will to live. Lula offers the girls a hope to return back to their former lives, but the cost involves showing them the grisly truth.
Hervé recalls his mother's death when he was a child. He goes to visit Dr. Bender to check on the health of both Anna (his sister) and Mireille (his cousin). Mireille suddenly falls ill and is diagnosed with the same symptoms his mother possessed at the time of her death. Knowing that Mireille won't live long, Hervé goes to see his grandfather to request to attack that "group", but he is ordered to wait. That night, Hervé, unseen, watches the four girls killing another beast man and in disgust silently reproaches the viciousness of their act.
A time of relative peace has come, allowing Kate, Rachel, Rose, and Claire to know each other better and coexist together. As they have dinner at Claire's home, Kate comments how fortunate they have been to receive a chance to return to their normal lives. Claire replies that even if their lives are not what they used to be, all of them are friends to each other that spend time and have fun together.
Unable to find a job to pay for the rent, Claire reluctantly goes to her brother to ask him for money. Rose and her siblings visit their hospitalized mother, while Rachel bumps into her school teacher by accident and both have a friendly chat at a cafe. Meanwhile, Kate spies upon Lula and JC but is soon spotted by Lula. Kate inquires about the mystery of their new bodies to which Lula replies that Lize's and their bodies are being targeted to be used for reproduction for the sake of a certain family clan's survival.
Lize wakes up in a clinic room, and Dr. Bender informs Hervé's grandfather that her bodily functions are in healthy condition. Soon, Hervé visits his grandfather and accuses him of forsaking Mireille and Anna's lives, granting medical priority to Lise instead. At daytime, Lula contacts the girls to survey their next target- a seemingly normal businessman. Claire cannot believe an innocent man could become their enemy, until all are ambushed by the same man transformed into beast state at night.
Emma introduces Emilio, her fianceé, to Kate. At school, Luke accuses Rachel of having an affair with the teacher, prompting her to breakdown in tears. Claire relunctantly returns to her former employer at the hamburger shop but finds out that he has truly missed her. The student council attempts to kick out Kate from Grace, but Paula won't allow it. Hervé meets with Emilio, who is also a clan member, and warns him to break up with his girlfriend because her younger sister is one of their enemies or else fear backlash from the head clan.
Hervé persuades Emilio to have Emma bring Kate for a day out with them, and later he volunteers to walk Kate home together alone. His intentions are to get closer to Kate in order to understand his enemies and find out who is leading the girls. Kate feels unsure whether or not she might actually feels something for Hervé. Meanwhile, Hervé has started to tease and emotionally abuse Lize in order to force her to regain her memories.
Complications among the girls' teamwork arise during their next sleepless mission when Rose's fear to take part in the fights almost put in danger Rachel's life. As a result, Kate herself finds out the hidden abilities within her new body and Rose goes to see her for training to become stronger. Hervé starts dating Kate and cynically informs Lise he's doing so as a deliberate act to torment and spurn jealousy on her, but he receives a phone call from Anna to return to the hospital because Mireille is agonizing.
Paula has a brief meeting with the school principal to discuss about telling the truth to Kate, but the principal speaks that they are not to take action but only to protect her. Later, Kate learns that Hervé lied to her the previous date, but soon he meets with her to tell her the truth that his cousin fell sick and had to lie to Kate so as not to panick her. Rose learns how to trigger the special abilities in her body to save Rachel from certain death.
Hervé breaks down after helplessly witnessing Mireille getting slain once she dies and the curse consumes her. He attempts to kill Lize but cannot bring himself to do so. Next morning, Hervé shocks Kate disclosing that he knows she has been killing all those men. Afterwards, Kate gets interrogated by the police, because an anonymous parcel was addressed to them containing Lise's diary. That afternoon, the school principal summons a meeting with Lula and the head of their clan to discuss about the appearance of the diary and making invesigations on the people surrounding Kate, starting with her boyfriend, Hervé.
Hervé contacts Kate to meet him at the docks on night. At the requested meeting, Hervé discloses that she and her friends have been fighting for an organization called Animus. Ages ago, members of said organization stole one of the two Cursed Tomes which were possessed by the ancestors of Herve's clan, casting on the latter the curse which is inherited through generations. A violent fight breaks out when both JC and Lula arrive, bringing along reinforcements from Animus, as well as Emilio to help Hervé. The Headmistress addresses Hervé and his clan as the Doral, cursed monsters which are destined to die.
During the previous fight, Hervé killed JC, who was Lula's brother. Kate visits the school's teacher to confirm the truth that Hervé disclosed to her, that all four of them will eventually lose all of their memories and live forever in their ageless bodies. She tells the truth to Claire, Rachel and Rose. Meanwhile, Hervé is put to jail on orders of his grandfather for his disobedience, but after apologizing for what's been lost, he convinces Emilio to bail him out.
Hervé kidnaps Lise with Emilio's help and brings her to the sea. There, he apologizes to her and leaves. However, one of the officers has been following them and finds Lize walking into the water, but he is shot by Hervé's family when he catches up to Lize. After a visit at Lise's tomb and a chat about how to live their lives in the future, the four girls decide not to fight tonight and Lula makes their job instead. Later, JC is buried.
Inspector Neil dials Kate to inform that he saw Lise alive but on custody of a group of men. As he is following the vehicle transporting Lise while at Brooklyn Bridge, it loses control and crashes. Meanwhile, Kate does a search on Vivaleo Pharmaceuticals and phones Hervé to demand Lise's return, but the conversation is cut when a group of Dolore men kidnaps Kate and brings her to Dr. Bender.
Hervé and Emilio infiltrate Vivaleo Pharmaceuticals as well as Lula, Rachel, Rose, and Claire. Emilio creates a commotion in the lab to help Kate and Lise escape, but Kate won't go because she deems him a liar for deceiving her sister, Emma. Hervé goes to find Anna, but she is held in custody by his grandfather, Lauro.
It is Christmas Eve, and each of the girls spend the night in company of their loved ones. As the sun rises in the morning, all four bid their last farewell and gather at the school to prepare for the upcoming battle.
The bridges connecting Roosevelt Island are destroyed by a set of explosions to cut all accesses to the mainland. War erupts among the Animus and Dolore, as casualties arise from both sides. Hervé, with Anna in his arms, breaks into the school grounds. The four girls follow in pursuit and find Lise at the school.