In this, the first of the seven books which make up the Chronicles of Narnia, we meet the four main (human) characters, Peter, Susan, Edmund, and Lucy. They have been sent to the country home of an old professor to escape the air-raids in London during WWII. The professor is a sweet old man, and the children are pretty much left to themselves to explore the large and mysterious house. And since it is raining the day after their arrival, their explorations begin that day. In one room is there is nothing but a large wardrobe—boring to all but the youngest, Lucy, who is enticed to enter it. Carefully leaving the door open, she walks through rows of coats, to find that their furry softness has become rough, and her feet are crunching on something cold. She emerges into the light, and sees, not the large room where she had entered, but a snowy woods with a lamp-post. And she meets someone very soon, whose name is Mr. Tumnus. But he isn't human, he is a Faun. He asks her if she is a Daughter of Eve.
Being quite friendly, he invites Lucy to his snug cave, where he prepares a nice little meal. Then he plays his flute and she becomes mesmerized. But he cannot do what he must do. He is in service to the White Witch, Jadis, to turn in any Daughters of Eve or Sons of Adam. She does not want humans in Narnia, and we soon find out why. And especially these four children.
He tearfully confesses to Lucy how he has betrayed her, then escorts her back to the lamp-post, where she enters the wardrobe, calling to her siblings that she has returned. They look at her oddly, since she was only gone a few minutes, not hours as she claims. She tells them of Narnia, and they think she is making up stories, but when she insists she is telling the truth, they begin to worry. She even shows them the wardrobe, but all they see now is a back wall.
But later in the day, as it is still raining, the children decide to play hide-and-seek. But this time it is Edmund who enters the wardrobe and finds himself in Narnia now. However, he is not met by the Faun, but by the White Witch, Queen Jadis, herself, She offers him Turkish Delight, his favorite candy, in exchange for information, which he freely supplies, including telling on his sister and Mr. Tumnus, who is now in big trouble because he has not followed the Queen's commands. He is allowed to return to the wardrobe, too, with orders to bring his siblings to her, upon which he will be made a prince, then king.
As he is approaching the wardrobe, he sees Lucy who has also entered Narnia, Now he must believe her, because he has experienced it himself. They begin to talk of the White Witch, and Lucy tells him how terrible she is and all the horrible things she has done to the creatures of Narnia, who are not people. He tells her Fauns lie, and begins to feel sick, probably from a combination of knowing in his heart he has betrayed his sister and her friend, and also knowing he will have to admit to his siblings he was wrong about Lucy (because it was he who made fun of her the most.) Plus he has eaten too much enchanted Turkish Delight.
But Edmund is a little beast, and when Lucy excitedly tells Peter and Susan that she and Edmund were both in Narnia, he laughs and says that they were playing make-believe. Lucy runs out of the room. The situation worsens until they finally go to the Professor with their problem. He asks them, which of the two, Lucy or Edmund, is more trustworthy? Who would more likely lie? And why is it so difficult to believe in a magic doorway?
So on the day the housekeeper, Mrs. Macready, is taking people on tour of the house, the children are advised to stay out of the way. And it is finally now that they all end up in the wardrobe, and enter Narnia together.
But something terrible has happened. Mr. Tumnus has been placed under arrest by the grey wolf, Fenris Ulf, the Captain of the Secret Police of Queen Jadis. The four children are led by a bird to Mr. Beaver, who takes them to his dam. Mrs. Beaver prepares a meal and updates the children on the state of things in Narnia. The White Witch has cast a spell so it is always winter but never Christmas. She turns those who oppose her into stone statues, and all live in fear of her. But something has changed. Aslan is returning to Narnia. He is more powerful than Jadis—much more. He is a lion. They must go meet him. They suddenly realize Edmund has disappeared. He has betrayed them once again, and is on his way to Jadis to tell her what he knows. They quickly prepare to flee.
But something is changing. They hear trickles. The ice and snow are beginning to melt. As they walk, flowers suddenly burst into bloom.
Meanwhile Edmund has reached the Queen's castle. But instead of welcoming him with the promised Turkish Delight, she is incensed that he has returned without his siblings. They try to catch up with the Beavers and the other children on their way to meet Aslan, but the sledge can no longer travel because the snow is melting so fast.
And that's all I will tell you. You have to read the books yourself, if you haven't already, to find out why the two Daughters of Eve and Sons of Adam are so important to Narnia, and such a threat to the White Witch.