The heir to the vast Zetes Corporation, Arthur, has become a complete misanthrope. Due to a fierce inheritance battle, his jealous stepbrother Maynard constantly sends assassins to kill him, leaving Arthur unable to trust any human being. His solution is to request a perfect, emotionless robot maid. However, due to technological limitations, his loyal butler Roy cannot acquire such a machine. Instead, he finds the next best thing in Marie, a destitute former martial arts prodigy whose naturally expressionless face makes her perfect for the deception. Desperate for money, Marie agrees to pose as a mechanical maid while secretly using her incredible strength to protect Arthur from the endless stream of threats.

The story follows this precarious lie as it unfolds within Arthur's lavish mansion and the halls of his prestigious academy. Marie must flawlessly perform her duties as a robot while hiding her very human reactions, a task made nearly impossible because Arthur, who despises people, has a bizarre habit of doting on inanimate objects, including his new maid. As Arthur showers his robot with affection, Marie finds herself struggling with growing real feelings for him. The central conflict balances this heartwarming romantic comedy with the constant danger of assassination attempts from Maynard and other enemies, as well as the ever-present risk of Marie's true identity being discovered, a secret that would mean instant dismissal or worse.

The original series focuses on Marie and Arthur as the primary couple. Arthur is an earnest young man who hides a kind and caring nature beneath his cold, distrusting exterior. Marie is stoic and incredibly skilled in combat, but internally she is flustered by her master's attention and the stress of her deception. Key supporting characters include Roy, the knowing and scheming butler; Maynard, the resentful stepbrother and primary antagonist; and Noah, a skilled assassin who becomes fixated on Marie. Later in the narrative, the robot Marie 2 is introduced as a genuine mechanical maid, creating a direct foil for the human Marie and adding to the complications.

Notable narrative arcs in the original story include the initial establishment of the deception, where Marie must prove her worth while dodging Arthur's curious questions about her supposed robotic functions. A significant arc involves the introduction of Marie 2 and the threat of Marie being replaced. The story also features a date sequence where Arthur activates girlfriend mode on his robot maid, leading to increasingly intimate and awkward moments. The constant assassination attempts from Maynard and later the romantic interference from Noah create ongoing tension. The narrative builds toward the inevitable liar revealed moment, where Arthur discovers Marie is human, a development that forces both characters to confront the truth of their feelings.

Following the conclusion of the original six-volume series, the story continues in the sequel, Mechanical Marie+. This installment picks up after the events of the first manga, further exploring the evolving relationship between Marie and Arthur as they navigate life together now that the initial secret is resolved. The sequel ran from April 2025 to December 2025 and is collected into a single volume, focusing on the aftermath of their unconventional beginning and the new challenges that arise from their established bond.