Detectives Nick O'Malley and Kate Benson work in Special Unit 2, a secret precinct of the Chicago Police Department whose sole charge is to protect the city's citizens from Links, a malicious paranormal species that is the missing link between man and beast.

A group of aliens arrive on Earth as part of an advance invasion force. Their mission is to kidnap a small sample of humans and run them through a series of rigorous tests to determine if they would make suitable slaves. Harper, one of the alien experimenters, has become frustrated with her planet's endless cycle of conquest and extermination and begins to feel some sympathy for her human captives. While the prisoners see a glimmer of hope in the compassionate alien, will it be any use in stopping the enormous attack fleet just a few days from Earth?