James is a 17-year-old who believes he is a psychopath. He kills animals as a hobby, but grows bored of the practice. He decides he wants to try killing a human. He settles on Alyssa, a mouthy, rebellious 17-year-old classmate with issues of her own. She proposes they run away together, hoping for an adventure away from her turbulent home-life, and James agrees with the intention of finding an opportunity to kill her. They embark on a road trip across England, and begin to develop a relationship after a series of mishaps.