In the London suburb of Finchley, the Pevensie children, Peter, Susan, Edmund and Lucy, are endangered by a Second World War attack of German bombers. They are then evacuated to the country home of Professor Digory Kirke, who is not accustomed to having children in his house, as Mrs Macready, the strict housekeeper, explains.
While the Pevensies are playing hide-and-seek, Lucy discovers a wardrobe and enters a wintry fantasy world called Narnia. Seeing a lamppost, Lucy encounters the faun Mr. Tumnus, who explains the land she has entered and invites her to his home. He puts Lucy to sleep by playing a lullaby on his flute. When Lucy wakes up, she finds Tumnus grieving, and he explains that Jadis, the White Witch, has cursed Narnia and it has been winter for 100 years. If a human is encountered they are to be brought to her. Tumnus cannot bring himself to kidnap Lucy, so he sends her home. When she returns to Professor Kirke's house, hardly any time has passed in the normal world; her siblings do not believe her story, and when they look in the wardrobe it has a normal back.
One night, Edmund follows Lucy into the wardrobe. He enters Narnia as well, and after searching for Lucy he meets the White Witch, who claims to be Queen of Narnia. She offers him Turkish Delight as well as the prospect of becoming king and having power over his siblings if he brings them to her castle. After she departs, Edmund and Lucy meet again and return; Lucy tells Peter and Susan what happened, but unfortunately, Edmund lies. Professor Kirke talks with Peter and Susan and suggests she is telling the truth, though they are unconvinced.
While running away from Mrs Macready after accidentally breaking a window, the four siblings retreat to the wardrobe and enter Narnia. They discover Mr. Tumnus has been taken by the Witch, and meet Mr. and Mrs. Beaver, who tell them about Aslan. According to the beavers, Aslan intends to take control of Narnia from the Witch. The four must help Aslan; it has been prophesied that if two sons of Adam and two daughters of Eve sit in the four thrones, the White Witch's reign will end.
Edmund sneaks off to visit the Witch. When he arrives at her castle, she is angry that he did not deliver his siblings. The Witch sends wolves to hunt down the children and the beavers, who barely escape. Edmund is chained in the Witch's dungeon, where he meets Tumnus. The Witch demands that Edmund reveal where his siblings are. After Tumnus claims that Edmund does not know anything, The Witch tells Mr. Tumnus that Edmund betrayed him, then turns Tumnus to stone.
While Peter, Lucy, Susan and the beavers travel, they hide from what they believe to be the White Witch. It is really Father Christmas, a sign that the Witch's reign is ending. Father Christmas gives Lucy a healing cordial, a drop of which will bring back to life anyone injured, and a dagger to defend herself. Susan receives a bow and arrows and a magical horn that will summon help when blown, and Peter a sword and shield.
After evading wolves led by Maugrim, the group reaches Aslan's camp. Aslan is revealed as a huge and noble lion who promises to help Edmund. Later, two wolves ambush Lucy and Susan. When Peter intervenes, Maugrim attacks him, and Peter kills him. Some of Aslan's troops follow the other wolf to the witch's camp and rescue Edmund. Peter is knighted by Aslan.
The White Witch journeys to Aslan's camp and claims Edmund, but Aslan secretly offers to sacrifice himself instead. That night, as Lucy and Susan covertly watch, Aslan is killed by the White Witch. In the morning he is resurrected because "there is a magic deeper still the Witch does not know". Aslan takes Susan and Lucy to the Witch's castle, where he frees the prisoners that the White Witch turned to stone.
Edmund persuades Peter to lead Aslan's army to fight the White Witch's forces. To stop the Witch from attacking and killing Peter, Edmund attacks the White Witch and destroys her wand, but is gravely wounded by her. As the Witch fights Peter, Aslan arrives with reinforcements and kills her. After Edmund is revived by Lucy's cordial, the Pevensies become Kings and Queens.
Fifteen years later, the Pevensie children have grown into young men and women. While chasing a white stag through the forest, they encounter the lamppost that Lucy saw on her first trip to Narnia. They make their way through trees, arriving in the wardrobe at the same time and day they left, becoming children again. Lucy later attempts to return to Narnia via the wardrobe, but Professor Kirke tells her he has tried for many years, and they will probably return to Narnia when they least expect to.

Question: What was Lucy doing  when she found the wardrobe?
Answer: Playing hide-and-seek
Alternative answer: playing hide and seek