Fifty years after humanity won a war against an alien invasion, the world is slowly rebuilding, but the remnants of the conflict remain in the form of space pests and space varmints, dangerous creatures left behind by the extraterrestrials. Society has adapted by creating private extermination companies to deal with these threats. Eighteen-year-old Sumire Ichigaya works for one such company, Saotome Busters, performing grueling, low-paying work to pay off the massive debts left by her absent father while protecting her younger brother. Her life takes a drastic turn when she is abducted by a UFO and forcibly fused with a powerful and malicious space varmint, which she names Duskin. The fusion grants Sumire the ability to transform into a monstrous creature, a power that attracts the attention of the Raiden Corporation, a massive conglomerate that has repurposed alien technology for energy and military applications. After a confrontation with Raiden agent Sayama Hazuki, Sumire and Duskin are forcibly separated, leaving Sumire with the ability to transform into the varmint at will, while Duskin is left as a mostly powerless but still sentient and demanding creature.

Sumire is recruited into Raiden's special anti-varmint squad, the Rai Go Special Task Force, receiving a high salary and company housing in exchange for using her dangerous abilities. The core dynamic of the story is the odd-couple relationship between Sumire, a hardworking, financially struggling, and emotionally repressed young woman, and Duskin, a violent, selfish, and cunning alien creature who initially despises humanity and constantly tries to manipulate Sumire into letting him take control of her body. Over time, Duskin begins to develop a protective instinct toward Sumire, prioritizing her safety over his desire to kill humans. The plot follows Sumire as she adjusts to her new life at Raiden, struggles to control her powers with the help of Hazuki and other squad members, and faces threats from hostile varmints, competing military corporations such as Red Rock, and mysterious alien forces that seek to reclaim the technology fused with Sumire. The story balances explosive action sequences with comedic interactions, including a notable arc where Sumire's hidden talent for gambling, specifically mahjong, comes into play. As the narrative progresses, the mystery behind the full dimension jacket, the source of Sumire's transformation ability, deepens, revealing that a higher power may have intentionally given her this power for unknown reasons, drawing the attention of allies and enemies alike.