How do Meg and Charles Wallace meet Mrs Which?

Meg and Charles Wallace meet Mrs. Which while they are on a quest to find their father, who has gone missing. They are guided to her by the mysterious and powerful Mrs. Whatsit and Mrs. Who, who introduce them to Mrs. Which, the most formidable of the three celestial beings. Mrs. Which appears as a shimmering, ethereal presence, emphasizing her ability to transcend time and space, and she plays a crucial role in helping the siblings understand their journey and the challenges they will face.