"Blue Drop" is a science fiction manga series that intertwines themes of alien invasion, human relationships, and personal identity. The narrative is set in a world where Earth has been covertly infiltrated by an alien race known as the Arume. These beings, originating from the planet Kentauros, possess advanced technology and a matriarchal society. Their mission on Earth is to gather intelligence and prepare for a potential invasion, but their presence remains largely hidden from humanity.

The story centers on Mari Wakatake, a high school student who transfers to the prestigious all-girls Kaihou Academy. Mari is initially portrayed as a reserved and introspective character, struggling with fragmented memories of a tragic incident from her childhood. Her life takes a dramatic turn when she encounters Hagino Senkouji, a mysterious and aloof classmate who harbors a significant secret: Hagino is an Arume agent tasked with observing and interacting with humans. Hagino's true identity is concealed by her human appearance, a result of advanced alien technology.

As Mari and Hagino grow closer, their relationship becomes a focal point of the narrative. Mari's curiosity and determination to uncover the truth about her past lead her to discover the existence of the Arume and their plans for Earth. Hagino, on the other hand, begins to question her loyalty to her mission as she develops genuine feelings for Mari and other humans. This internal conflict is further complicated by the presence of other Arume agents, each with their own agendas and perspectives on humanity.

The plot delves into the complexities of interspecies relationships, exploring themes of trust, betrayal, and the moral implications of colonization. The Arume's advanced technology, particularly their ability to manipulate memories and emotions, plays a crucial role in the unfolding events. As Mari uncovers more about the Arume and their intentions, she becomes a key figure in the resistance against their plans, inadvertently drawing Hagino into a precarious position between her duty and her newfound empathy for humans.

The manga also examines the broader societal impact of the Arume's presence, touching on issues of power dynamics, cultural assimilation, and the ethical dilemmas faced by both humans and aliens. The narrative is punctuated by moments of tension and conflict, as well as quieter, introspective scenes that highlight the emotional struggles of the characters.

"Blue Drop" is characterized by its intricate plot, well-developed characters, and thought-provoking themes. The series balances its science fiction elements with a deeply personal story of connection and self-discovery, making it a compelling read for fans of the genre.