The narrative picks up with the protagonist, Search, a 15-year-old heavily armored warrior with memories of a past life as an assassin. His primary motivations in this life are an obsession with large breasts and bikinis, leading him to choose his profession solely because it allows him to wear bikini armor. Despite his unusual priorities, Search is a capable and charismatic leader, known for his bright personality, wide social circle, and a tendency to tease his companions. However, his assassin past surfaces when he shows no hesitation in killing those who threaten his friends. He also possesses unexpected domestic skills, being an excellent cook and enjoying hot spring hopping.
The story progresses through several key events. Search, along with his party, embarks on a journey to the Dark Continent to find the missing Aimiya Donovan, a 16-year-old Wind Sage who was once chosen as a hero. They become embroiled in a war on the continent, where Search's leadership helps the allied kingdom achieve victory and rescue Aimiya. However, a temporal anomaly sends Search back to his previous life's world.
The party consists of several other members with unique backgrounds and abilities. Lilia, a 17-year-old cat beastman archer, is a skilled fighter proficient in both long and close-range combat, acting as a voice of reason for the group. She shares Search's desire for larger breasts but has had no success in her own growth. Lize, a one-year-old cursed swordswoman, is a detached entity created from a separation of Rudel, a former party member who was transformed into a woman. Lize inherits Rudel's memories and possesses monster-like skills, preferring full-body armor. Vii, a Medusa and direct disciple of the Demon Lord Soleil, is a Holy Arts user with powerful racial skills like petrification and immense strength, though she struggles with fine motor skills. She harbors romantic feelings for Search, which are sometimes reciprocated in drunken moments. Marshan, an attack mage who is secretly a high elf queen, is a master of magic, particularly healing, and is known for her difficult personality and a penchant for trouble. Eliza, a 16-year-old maid and heavily armored warrior, is fiercely loyal to her mistress, Rifaris, and initially clashes with Search due to her protective nature. Ekater, a pharmacist, was raised by an assassin's organization but defected to serve Aimiya, becoming Search's slave after being captured. Nya, a Moon Witch and former werewolf chief's daughter, joins the party after being ostracized for her inability to transform, fighting with illusion magic and a broom.
A significant turning point occurs after the "Beast of Despair" incident. Lize accompanies Search to the Dark Continent, changing her weapon and attire, and develops feelings for him after he saves her from an illness. Vii leaves the party to pursue politics but is later sent to the Dark Continent by Rifaris and reunites with Search, also being transported to his past life's world. Marshan leaves the party after an incident involving merfolk but continues to aid Search from afar. Eliza joins the party after the "Beast of Despair" but returns to Rifaris later. Ekater becomes Rifaris's subordinate but remains Search's slave. Nya is caught in a temporal distortion with Search and Vii, ending up in Search's past life's world and showing signs of affection for him.
The narrative then shifts to Search's past life, detailing his brutal upbringing as an assassin, where survival meant killing. He developed a deep longing for a life where he could enjoy simple pleasures, particularly seeing women in bikinis, a stark contrast to his own physically hardened body. After a successful but fatal mission against his organization, he dies with the wish to be reborn with a large bust and wear a bikini.
He awakens in his new life as a baby, retaining his past life's memories and his peculiar desires. He is adopted into an orphanage and discovers he is in a world akin to an RPG, complete with a status screen. Despite lacking any cheat abilities, he resolves to train rigorously, aiming to become a speed-focused warrior, surpassing his past assassin skills. His ultimate goal remains achieving a large bust and wearing a bikini, though he is dismayed to find bikinis do not exist in this world. He dedicates himself to physical training and a unique "bust-enhancing massage" routine.
Years pass, and Search, now known as Shaa, has become exceptionally fast, though her strength remains low. She discovers a latent magical ability to manifest blades from nothing, a skill the orphanage director, Hilda, a former A-rank adventurer known as "Flying Sword," recognizes and decides to nurture. Hilda arranges for Shaa to attend a guild academy to further her magical training.
Shaa then embarks on her journey, equipped with basic supplies and a desire to become an adventurer. She encounters goblins and bandits, showcasing her assassin-like combat style and her unique magical ability. She also grapples with her low stamina and the mystery surrounding her magic. Her ultimate goal is to reach A-rank and be able to wear a proper bra, hinting at her continued pursuit of her bust-related aspirations.
Finally, Shaa arrives at "Upriz Armor Shop" in Hagifield, a renowned establishment specializing in bikini armor. She seeks a custom-made bikini armor that will allow her chest to move naturally during combat, a request that even the legendary craftsman, Head, agrees to fulfill.
