The Frennatti, slug-like aliens, arrive on Earth. Their plan is to colonize. However, in the interest of the environment, they will kill one human for every Frennatti who arrives to maintain balance.
Colin Backus, 17, decides to ignore his summons, partly for the love of his girlfriend, Riley. The Frennatti plan to kill every person on Earth as an example to never defy them. The President convinces their leader, Klorn, to give him 48 hours to find Colin.
After trying to escape, a very-injured Colin is located and brought aboard the aliens' flagship - where he is healed. The Frennatti want to understand what made him run before exterminating him. A scan of his mind introduces them to the emotion of love. It worries them. It is illogical and affects everyone differently. Will others defy them? Could it poison their own race?
To see the emotion firsthand, they put Riley in danger and give Colin the opportunity to save her. At the risk of his own life, he does.
The Frennatti realize they cannot stay on Earth because of love. They leave. However, as a warning to other travelers who might become infected by love, they set up a warning buoy and decide that any Earthlings who venture beyond the moon must be destroyed to prevent further infection.