Mark is a spy who returns to West Berlin after a shadowy espionage mission, only to discover that his wife, Anna, is seeking a divorce. She refuses to disclose her reasons, firmly asserting it has nothing to do with another man. In a state of despair, Mark concedes to relinquish the apartment and custody of their young son, Bob, to her. After a self-destructive binge, he visits the apartment, finding Bob neglected and alone. When Anna comes home, Mark remains with Bob, unwilling to leave them alone together, as he tries to mend their fractured relationship. However, Anna abruptly leaves in the middle of the night.
Soon after, Mark receives a distressing phone call from Anna's lover, Heinrich, who informs him that she is with him. The following day, Mark meets Bob's teacher, Helen, who bears an uncanny resemblance to Anna—with striking green eyes. The situation escalates as Mark confronts Heinrich, only to be beaten, while Anna suffers abuse at home and ultimately flees. During a subsequent, intense argument, both Mark and Anna end up injuring themselves with an electric knife.
Determined to uncover the truth, Mark hires a private investigator to tail Anna and unearths that she has been secretly maintaining a second apartment in a run-down building. Terrifyingly, the investigator stumbles upon a grotesque, tentacled creature in her bedroom. In a fit of rage, Anna murders him with a broken bottle. The investigator’s lover, Zimmerman, visits the flat, only to discover the horrific creature and his partner's lifeless body. In the struggle that follows, Anna violently slays Zimmerman before embarking on a path of increasingly erratic behavior. She reveals to Mark the traumatic experience of a miscarriage she endured in the subway while he was away, detailing the disturbing aftermath as she bled profusely.
As events spiral further out of control, Heinrich visits Anna's second apartment and is horrified to find both the creature and a collection of dismembered body parts in her refrigerator. After a violent confrontation, Heinrich escapes, bleeding, and begs Mark for help. Arriving at Anna's apartment first, Mark uncovers the gruesome evidence only to find the creature has vanished. Later, he meets Heinrich at a bar, where he murders him under the guise of an accidental death, staging it in a bathroom stall. Mark then sets fire to Anna's apartment before stealing Heinrich's motorcycle to flee the chaos.
Upon returning home, he encounters Anna's friend, Margie, mortally wounded and bleeding as she steps out of the elevator. After her death, Mark drags her body inside, where he and Anna engage in a frenzied sexual encounter in the kitchen. Consequently, he devises a plan to conceal Margie's demise, but then walks in on a shocking scene: Anna in bed with the creature, repeatedly crying "Almost!" in torment. Shortly after, Mark is called by Heinrich's mother, who queries about her son. When he meets her, she tragically takes her own life by poisoning.
The next day, as Mark roams the city streets, he faces pressure from former business associates to rejoin them, but he is evasive. Returning to Margie's apartment, now swarming with police and his prior employers, he creates a distraction to help someone—possibly Anna—escape in his car. However, he is shot in the ensuing chaos. In an attempt to flee on the motorcycle, Mark suffers a catastrophic accident, crashing into a building while being pursued by Anna, police, and former associates. In a dramatic climax, Anna unveils the creature, fully transformed into Mark’s doppelgänger. In a moment of desperation, Mark raises his gun, but he and Anna fall victim to a barrage of police gunfire. In their final moments, bloodied and fading, Anna uses Mark’s weapon to take her own life, collapsing in his arms as he leaps through the stairwell to his own demise. The doppelgänger escapes to the roof.
In the aftermath, Helen is at the flat babysitting Bob when the doorbell rings. Despite Bob's urgent plea not to answer, she disregards him. Outside, the chaos of sirens, planes, and explosions fills the air. In a moment of dread, Bob rushes to the bathroom, where he is found floating face-down in the bathtub. The chilling silhouette of Mark's doppelgänger is visible through the frosted glass door, as Helen stares on, her green eyes gleaming in the gloom.
