"The Door into Summer" is a manga adaptation of the science fiction novel by Robert A. Heinlein, reimagined by artist Hiroshi Mori. The story is set in a near-future world and centers on Dan Davis, an engineer and inventor who creates advanced household robots. Dan’s life takes a dramatic turn when his business partner, Miles Gentry, and his fiancée, Belle Darkin, betray him. They manipulate him into signing over his company and place him in suspended animation, a process known as "cold sleep," under false pretenses.

Dan awakens 30 years later in the year 2000, disoriented and stripped of his former life. Determined to reclaim his stolen inventions and seek justice, he navigates a world transformed by technological advancements. Along the way, he encounters Ricky, a young girl who bears a striking resemblance to his former fiancée, Belle. Ricky’s connection to Dan’s past becomes a pivotal element in his journey, as he uncovers the truth about his betrayal and the passage of time.

The narrative explores themes of loyalty, betrayal, and the consequences of technological progress. Dan’s ingenuity and resilience drive the plot as he devises a plan to travel back in time using a combination of cold sleep and time travel. His goal is to prevent the betrayal that upended his life and to reunite with Ricky, whose fate becomes intertwined with his own. The story blends elements of hard science fiction with emotional depth, examining the human cost of ambition and the enduring power of hope.

The characters are defined by their relationships and motivations. Dan Davis is a pragmatic and resourceful protagonist, driven by a desire to right the wrongs done to him. Miles Gentry and Belle Darkin serve as antagonists, embodying greed and deceit. Ricky, with her mysterious connection to Dan’s past, adds a layer of emotional complexity to the narrative. The interplay between these characters drives the story forward, creating a compelling exploration of time, memory, and redemption. The manga retains the core themes of Heinlein’s original work while presenting them in a visually engaging format, making it accessible to both fans of the novel and new readers.