In this pivotal chapter, Meg, Charles Wallace, and Calvin O'Keefe encounter Mrs. Which, one of the three mysterious celestial beings who guide them on their journey. The atmosphere is charged with a sense of urgency as they learn about the peril that Meg's father, Mr. Murry, faces.

The chapter begins with the children still grappling with the revelations from their previous encounters with Mrs. Whatsit and Mrs. Who. The three beings represent different aspects of wisdom and strength, and their presence signifies the weight of the task ahead. Mrs. Which appears in a somewhat ethereal form, emphasizing her otherworldly nature. She is described as being larger and more imposing than the other two, with a voice that seems to echo through space and time. This portrayal underscores her role as the most powerful of the three, capable of transcending the physical realm.

As they gather in a dimly lit room, Mrs. Which conveys the grave news that Mr. Murry is in danger, trapped on a dark planet called Camazotz, which is shrouded in evil. The children learn that he has been missing for some time and is being held captive by a malevolent force known as IT. This entity embodies a dark presence that seeks to control and dominate, reflecting the broader theme of good versus evil that permeates the story.

Mrs. Which provides insight into the nature of the universe, explaining the concept of a tesseract, a form of travel that allows them to traverse vast distances in space and time. This idea of folding space is pivotal for their journey, as it signifies that they will not merely travel through conventional means but will instead experience a reality where time and space can be manipulated. The notion of the tesseract introduces a fantastical element, highlighting the book’s exploration of scientific concepts along with its moral and philosophical themes.

The chapter also serves as a character development moment for Meg. She is filled with both fear and determination as she contemplates the rescue mission. Her feelings of inadequacy and self-doubt surface, but the urgency of her father's situation ignites a sense of responsibility and courage within her. Charles Wallace, with his exceptional intellect, offers insights that complement Meg's emotional struggles, showcasing the bond between the siblings. Calvin, ever supportive, brings an additional layer of friendship and loyalty to the group dynamic.

As Mrs. Which prepares them for the journey, she emphasizes the importance of love as a guiding force in their quest. This pivotal lesson sets the foundation for the children’s understanding of their mission: that love is a powerful weapon against the darkness they will face. The chapter concludes with a sense of anticipation and resolve, as the children are set to embark on a journey that will challenge them both physically and emotionally, reinforcing the book's central themes of love, courage, and the fight against evil.