As the girl begins to grow, she appears to be completely human. Once she reaches the age of puberty things begin to change. Scared for her life, the young girl breaks out of the controlled facility and runs. It is during this time that she hides on a train and with help from one of the conductors gets food.
While on the train trip the girl hibernates in some sort of cocoon where she grows into a woman within a few hours. Not knowing what has happened to her and still frightened the alien DNA takes over more and she begins to kill.
An insatiable mating drive forces the girl to seek out a male partner. With an increased need to have a baby, the alien creature searches for the perfect healthy man to get her pregnant. This leads to more deaths as she works her way through possible suitors.
Meanwhile, the government has launched a group of special people to track the hybrid and kill it before anymore lives are lost or before she can breed. After tracking her to a hotel, one of the group inadvertently sleeps with her allowing the alien to breed. After discovering who he was she kills him and again flees the scene. The rest of the team follow her and eventually manage to trap and kill the creature. The last scene of the movie shows that a rat from eating one of the creatures tentacles morphs into an alien creature and begins to attack another rat. Proving that the cycle still continues.