Lucky Luck, originally named Sachiko Tokisada, is a 14-year-old girl from the occupied planet Moad. She belongs to a resistance group fighting the Earth Federation, led by her elder brother Shiro. Facing her homeworld's imminent destruction by the Federation's planet-killing weapon, the Moby Dick, she desperately seeks Captain Ahab, the galaxy's greatest whale hunter.

To reach Ahab, she stows away aboard his spacecraft, the Lady Whisker. Disguising her identity and gender, she pleads to join his crew as an apprentice. To prove her worth, she undergoes a perilous trial: retrieving a coin from a well filled with dense, sinking liquid. Though requiring rescue by Ahab, she succeeds in retrieving the coin, demonstrating her determination. This earns her a probationary position under young crew member Atre.

As an apprentice, she adapts to life among the Lady Whisker's diverse crew, learning to hunt resource-rich derelict spaceships called "whales." Her bravery and resourcefulness gradually win the crew's respect. When her gender is revealed, she confesses her true purpose: pleading for Ahab's intervention to save Moad. Ahab agrees, motivated primarily by his own vengeful obsession with the Moby Dick, which destroyed his left eye and leg years earlier during an attack that harmed civilians.

Her journey shifts from a stowaway focused solely on saving Moad to an integrated crew member, actively participating in spacefaring adventures and the escalating conflict with the Federation. Her presence influences crew dynamics, fostering a close friendship and potential romantic feelings from Atre. Her narrative perspective frames the story as recounted from her future viewpoint.

Throughout the conflict, her resolve is tested against Federation forces and the terrifying Moby Dick. Her arc culminates in her role in the final confrontation against this weapon, intertwined with Ahab's vengeance and Moad's fate.