Possible sabotage at a razor quartz mine gives Captain Joseph Darrow the final evidence he needs to prove that there is an unknown alien species stealing from sites across the galaxy. A man is killed at one of their raids, and his last word is 'butterfly'. What does it mean? Darrow's ship the Kestrel is sent to contact these 'Intruders', despite being small and ill-equipped for such a mission, and the allied species insist on sending representatives too, making it overcrowded. With them comes Anna, a mysterious injured woman, who can hear the Intruders' thoughts, but they terrify her. Tensions rise as the crew and representatives cope with the overcrowded quarters and encounters with Intruder ships. The finding of a crippled Intruder ship and offer of assistance gives Darrow the breakthrough he needs, but not before deaths among the crew and the Intruders. When the Intruders' planet is finally reached, the landing party are taken captive, and it is up to Anna to save the day.