A bachelor party becomes a savage fight for survival when the groomsmen unwittingly unleash a fabled predator upon the festivities.
Feeling excited about getting married, groom-to-be Jonah and his groomsmen depart for an epic bachelor party. However, as one thing leads to another, the four friends end up in a remote mansion which houses a private underground strip club for a select clientele with extreme tastes. And before long, Jonah sees first-hand the dark wonders of the secret nightclub as he finds enigmatic Lily singing a strange but entrancing song from the other side of the glass of his VIP room. Now, there is no turning back. But is beautiful Lily a victim or a predator? An angel or a demon?—Nick Riganas
Mr. Nyx, proprietor of underground occult club 50 North/40 West, and Sheriff Boone investigate a church where a cult was slaughtered after summoning the young siren Lilith. Lily kills Sheriff Boone. Nyx captures the creature by placing a binding bracelet on her ankle.
Eva says goodbye to her fiancé Jonah as he departs for a bachelor party with his brother Mac, best friend Rand Wilson, and Elliott Duncan. At a strip club in Garden City, Mac meets a mysterious man who tells him about a secret underground club that fulfills wild desires.
The four friends get high on mushrooms and follow the man to a remote mansion where the 50 North/40 West club is hidden. After they see several of the club's strange sights, Mr. Nyx appears and escorts the quartet to a private room.
Nyx sends Jonah alone into a hallway where Jonah finds Lily confined to a small room. Through the door's glass window, Lily sings a strange song that entrances Jonah.
As payment for Jonah's unique experience, Nyx asks that the other three men give him the fondest memories of their mothers. Bewildered, the trio agrees. Nyx has Mac, Rand, and Elliott look down and then tells them they are paid in full. Later, the three men notice that odd tattoos now appear on the backs of their necks. The club's bartender serves Elliott a drink with a live leech and soon after, Elliott begins acting paranoid and remorseful for a murder he didn't commit.
While Mac and Elliott wait outside with the car running, Jonah enlists Rand to help him free Lily. A pair of brother bouncers attempts to stop the rescue. As one of the bouncers fights Rand, Lily rushes Jonah into a bathroom and unlocks her ankle restraint with keys stolen from the bouncer. Jonah watches in horror as Lily sprouts a tail and devours one of the bouncers.
Jonah and Rand make it outside to Mac's car. Other club patrons also flee the house in panic as Lily goes on a rampage.
Lily flies after Mac's car and abducts Elliott through the passenger side window. Mac crashes the car. Rand is run over while trying to flag an approaching vehicle. The vehicle turns out to be carrying Nyx and the other bouncer, Barry, who capture Rand as Jonah and Mac flee into the forest.
Jonah realizes that Lily wants him specifically. He and Mac make it to a diner and tell their story to police officers O'Brien and Collins. However, the cops have the strange tattoos on their necks and report finding the two men directly to Nyx.
Upset over losing Lily, Nyx interrogates Rand and has Barry torture him.
Lily arrives at the diner and seduces the officer holding Jonah with her song. She then kills both policemen. In full creature form, Lily professes her attraction to Jonah by saying, "I like you." Mac sees Lily fly away with Jonah. Lily takes Jonah to a field where she sings her song and has sex with him.
Nyx has the bartender remove a sentient leech from her head that she uses to extract the torture experience from Rand's neck. Nyx then instructs the bartender to find Jonah and make him ingest the worm.
Jonah leaves Lily sleeping on the ground and makes it back to his hotel room on foot. There, he is confronted by the bartender, who forces him to drink the worm and relive Rand's pain. The bartender tells Jonah that Nyx wants Jonah to bring him Lily in exchange for releasing Rand.
Jonah meets Nyx at the church where Nyx first found Lily. Nyx gives Jonah the binding bracelet and tells him he is the only person who can get close enough to recapture Lily.
Lily comes to the church and starts attacking robed cultists. Jonah apologizes for leading her into Nyx's trap and seemingly makes an agreement for the two of them to go their separate ways. Lily nods and bolts for the cellar with Barry in pursuit. Jonah discards the binding ring.
Nyx attacks Jonah. Mac arrives at the church and beats Nyx off of Jonah. However, Nyx recovers and cuts Mac's throat. Nyx begins approaching Jonah as Jonah attempts to free Rand.
Having killed Barry, Lily returns to the altar and sings her song to entrance Nyx. She then thrusts the tip of her tail through Nyx's head, killing him.
Lily kisses Jonah before flying away. Jonah escapes the church with Rand and returns home to Eva.
On their wedding anniversary one year later, Jonah briefly thinks he hears Lily's call before having sex with Eva. Afterward, Jonah goes downstairs and finds Eva sleeping on the couch. Jonah realizes that the woman he actually slept with was Lily.
Lily comes downstairs. Jonah pleads with her to leave. Eva wakes and Lily prepares to attack. Jonah agrees to go with Lily if she spares Eva. Eva screams as she watches Lily fly into the sky with her husband.
