Constance Bonacieux serves as chambermaid to Queen Anne of France and rises to become her trusted confidante due to her unique knowledge of the Queen's secret relationship with the Duke of Buckingham. This position embroils her in dangerous secret missions on the Queen's behalf. In this adaptation, she is portrayed as the daughter of Monsieur Bonacieux, not his wife as in the original novel.

Her path crosses with D'Artagnan when he accidentally collides with her while riding. Though physically unharmed, the 16-year-old rejects the immediate romantic advances of the 15-year-old, perceiving him as immature. Her attitude evolves as D'Artagnan repeatedly rescues her from peril, proving her innocence when falsely accused and racing to save her after kidnappings. These acts of bravery gradually win her affection.

She faces multiple kidnappings and dangers orchestrated by antagonists like Milady de Winter and Iron Mask. An explosion leaves her in a coma, resulting in temporary amnesia upon waking; D'Artagnan remains deeply concerned and helps restore her memories. Despite frequent victimization, she displays agency, attempting a critical mission to England herself when the Musketeers are unavailable.

Her romantic relationship with D'Artagnan develops through challenges. She becomes angry with him at times and initially avoids his attempts to kiss her, though they eventually share a kiss. In the film sequel, their bond is tested when she mistakenly believes he is involved with another woman. After clarifying the misunderstanding, she supports him wholeheartedly and joins him in combat against their enemies.