A red object falls from the sky, hitting into a wooded area, and a deer in the woods is grabbed by a tentacle.Ruth and Harry, an unmarried couple in a six-year relationship, agree to pursue Harry's interest in camping together in the woods. They hike to a scenic overlook with water in the background where Harry proposes marriage. The anxiety-prone Ruth panics and rejects him, arguing that they talked about never getting married in the past. This conversation causes some tension. Walking in the woods the next day, they come across a dead deer covered in a black substance, which Harry guesses to be the result of an illness.Later, Ruth enters a cave and finds a blue puddle. Emerging from the cave, she tells Harry she has reconsidered and wants him to propose to her again. This time, however, when they reach the scenic overlook, she pushes him off the cliff; he falls on a rock and is apparently dead. Wandering in the woods afterwards, Ruth finds another couple who try to establish the cause of her disassociated state. An alien impostor in the guise of Harry appears and kills the couple.It is revealed that when Ruth found the blue puddle, she also saw Harry's dead body in a cocoon in the cave, and knew he had been replaced by an imposter. The alien who has taken Harry's place finds itself unable to kill Ruth because it absorbed Harry's love for her. Love is not an emotion of which the alien has any prior knowledge, and he finds it difficult to adapt.The alien takes Ruth to a beach and reveals his spacecraft, offering Ruth the chance to accompany him to another planet in order to escape the forthcoming invasion of Earth. Ruth stabs the alien in the chest and lures him into the water, where his blood attracts a shark, allowing Ruth to escape. The alien manages to kill the shark and pursues Ruth again, eventually rendering her unconscious and encasing her in a cocoon. The alien absorbs Ruth's personality and character, and imitates her appearance. However, it is unadjusted to the anxieties from which it now suffers as a result of this, and becomes incapacitated. The real Ruth, escaping the cocoon, overcomes the alien and smashes its head with a rock. After she flees, the alien's head is seen beginning to reform.Ruth finds her way back to the parking area. As she drives away, the alien speaks to her through the car radio, and red objects fall from the sky, signifying the broader alien invasion. (thanks to Wikipedia)