Chapter Four of A Wrinkle in Time is set on the planet of Uriel, the third planet around a very distant star, Malak. This is quite confusing to Meg, Calvin, and Charles Wallace; not only is the weather and the countryside different, but Mrs. Whatsit changes form, revealing her true self to be some sort of flying centaur.
The main problem with the children, Mrs. Who, Mrs. Which, and Mrs. Whatsit being on the distant planet is that it has happened in an instant. It is an impossible distance to travel, even at the speed of light. Mrs. Which explains that they have instead brought the two distant points together, rather than traveled between them. The book gets its title from this "wrinkling" of time and space.
The other important aspect of the planet Uriel is that is the first place the children see the dark cloud of the evil that their father is fighting.