His Dark Materials is a fantasy adventure book series. The story follows the adventures of the main characters - Lyra and Will - as they travel through different worlds and discover hidden secrets about the universe. They encounter different creatures and people on their journey, including witches and armored polar bears. Along the way, they learn about the mysterious energy called Dust, which is connected to all living things in the universe. The books show how people can be brave and how they can fight against evil to protect what is right.