Due to a crew shortage, 18-year-old Tabitha Enns is given work experience on board the Kestrel, part of the fast-response fleet of the Planetary Alliance for Cooperation and Trade (PACT). The fast-response fleet is a diplomatic and emergency service, and Tabitha is excited at the chance to get away from her home planet for the first time. She is shocked to find she is the only female on board and has to share a cabin.
Her first mission is to a razor quartz mine where there has been an explosion and a cave in. But the Kestrel’s captain, Joseph Darrow, is also asked to investigate possible sabotage to cover up thefts of the valuable razor quartz crystals.
Darrow's investigation gives him the final evidence he needs to prove that there is an unknown alien species stealing raw materials from sites across the galaxy. One of the miners is killed during an attempted theft, but why did the dying man say, 'butterfly'? Darrow’s superiors in PACT have their hands full with tension on the border, and do not see the urgency. Eventually Admiral Keever is convinced, but due to the crisis elsewhere, no ships are available for a mission to find the Intruders.
The Admiral orders Darrow to take the Kestrel, a small ship with a crew of eleven, and not equipped for such a task. To make it worse, at a briefing with the other Alliance species, they insist that a representative from each species should also go on the mission, putting the crew complement up to seventeen. Tabitha finds herself moved into a corner of Engineering, where Roy Stubbs works, who is attracted to her. Having never had a boyfriend, this is a problem.
Several crew changes are also necessary, so Darrow must undertake the most serious and potentially dangerous mission of his career with an overcrowded ship and untried crew – to look for energy beings who may be pillaging the galaxy.
As the representatives arrive, so does a mysterious woman, Anna, who has a head injury and amnesia. As they follow the trail, Anna shows that she can ‘hear’ the thoughts of the Intruders, and is a valuable asset, but where did she come from and who operated on her brain to give her this ability, which terrifies her? The questions add to the tensions between those on board.
Tabitha gets to meet her first aliens, but it doesn't go smoothly, and being out in space isn't what she expected. When an Intruder ship attacks an Altarian colony, Kestrel goes to their aid, and Tabitha helps with the cleanup and gets her first experience of another planet.
The finding of a crippled Intruder ship and offer of assistance gives Darrow the breakthrough he needs, and solves the ‘butterfly’ mystery, but not before deaths among the crew and the Intruders. When the Intruders’ planet is finally reached, despite a friendly reception, the landing party are taken captive, and it is up to Anna to save the day. Tabitha's future remains uncertain though, as she stowed away with the landing party to see the new aliens for herself.