It is the story of the four Pevensie children - Peter, Susan, Edmund, and Lucy, who accidentally stumbled upon Narnia, while staying at the home of elderly Professor Kirk.
Lucy was the first to venture into the old wardrobe and discover that it was a portal to another world - a magical world in which animals can talk; but this was also a dangerous world ruled by the evil White Witch, who had cast a spell on the land, ensuring that it was "always winter, but never Christmas".
Lucy returned to tell her siblings about her adventure, but she was met with skepticism. Edmund followed her through the wardrobe and discovered the truth, but he was weak and spiteful and was easily seduced by the White Queen's magic and promise of enchanted sweets. They returned later with their older siblings and learned that the lion Aslan was returning to Narnia after an absence of centuries. Aslan was held in great reverence by the Narnians and was destined to confront the White Queen and challenge her reign.
Lewis combines several different cultures in his narrative. He drew inspiration for his characters from the mythologies of the ancient Greeks, Romans, Norsemen, and other cultures and from the stories of Hans Christian Andersen. Even Father Christmas makes an appearance at one point. But the story also has many influences from Lewis's Christian faith. It is an allegory of Jesus Christ's sacrifice and resurrection to redeem the sins of others. Lewis does a remarkable job of suggesting Biblical stories without repeating or imitating those stories.
"The Lion, the Witch, and the Wardrobe" is a classic story that combines adventure, magic, family, action, religion, betrayal, forgiveness, and redemption in a very short space.
I never read this book as a child. I was in my 40s when I first experienced Narnia. Two decades later, I find it is still magical.