The small moon of Danegeld Four is bustling with pirate activity, and mercenary Samantha Omleth is hired to investigate their activities. Her stake in the mission is personal; after her parents died when she was a child, she had been raised by the natives, the ansaxo, and it was when she was when she had returned to her birthplace of Earth as a teenager that they were almost wiped out by the pirates.
Samantha quickly uncovers the hierarchy of the pirates, led by the robotic Thought Inbras. Among their number is Kauldus, the sadistic pirate who had originally killed Samantha's parents. She also discovers that the pirates are breeding extremely dangerous creatures with many strange properties called pheliapods, with intent of using them as weapons.
Finally, Samantha finds an artificial intelligence based on her adoptive father and mentor, Elder Omleth, who tells her of what happened in the invasion, including the death of the real Elder Omleth. At first she wants to take revenge on the pirates but she is urged away by Elder Omleth, who is designed to keep the ansaxo alive and wants Samantha to continue the ansaxo legacy where it is safe.
Before leaving, Samantha saves an infant pheliapod from torture, and the pheliapod starts to believe that Samantha is its mother. Samantha brings the pheliapod to Earth
for further study, naming it Gertrude, and over the course of several days begins to bond with it. However, her feelings are complicated by the fact that pheliapods can artificially manipulate the emotions of others, and she knows Gertrude could well just be using her. The pirates strike back, with Kauldus brutalising Samantha. She would have killed Samantha if not for the timely arrival of Horatio Walker, Samantha's employer, but she escapes with Gertrude in tow.
Despite her promise to her father and repeatedly being told not to bother saving Gertrude, Samantha returns to Danegeld Four again. She struggles with her own motives for returning, as Elder Omleth tries to drive her away and berates her for caring for the creatures that killed the ansaxo, while Gertrude manipulates her into trying to save her. Deciding that focusing on revenge is easier than making that choice, Samantha fights her way through the pirate ranks to Kauldus and after another brutal rematch she finally vanquishes Kauldus.
With Thought Inbras holding Gertrude captive, Samantha goes to Thought Inbras' lair, who intends to use the pheliapods to make her all-powerful. Samantha is forced to suffer the same pain Gertrude suffers at Thought Inbras' hands, and finally confronts the pirate leader. The weight of the battles and injuries she has sustained almost kill Samantha, and it is only when Gertrude sacrifices its life to save Samantha that she escapes.
Samantha, in a depressed state, meets with Elder Omleth again, who berates her for betraying the ansaxo. However, she realises that Elder Omleth is only an artificial intelligence with none of the real Elder Omleth's teachings. In a rage she destroys the AI and, in a dream state, earns Gertrude's forgiveness for abandoning it to chase Kauldus.
Now deciding that the other pheliapods cannot suffer the same way Gertrude did, Samantha returns to Thought Inbras and finally triumphs, dismantling the pirate operation in full and saving the pheliapods. She leaves Danegeld Four for good, committing herself to protect the weak where she may find them and stop letting her past dictate her present, and following Elder Omleth's teachings rather than his memory.
