"It is the Alethiometer. It tells the truth. As for how to read it, you'll have to learn by yourself."
In a parallel universe in which a person's soul resides outside the body in the form of an animal called a "dæmon", Europe is controlled by an authoritarian organization called the Magisterium. In Oxford, orphaned Lyra Belacqua, a ward of Jordan College, tells of the "Gobblers", who she and her friends hold responsible for the disappearance of several other local children. The college is visited by Lyra's uncle, Lord Asriel, who intends to present evidence which points to the existence of particles called "Dust" —an idea in opposition to official Magisterium doctrine. After Lyra saves Asriel from assassination at the hands of a Magisterial representative, he obtains funding from the college to mount an expedition to the far north in order to investigate the substance, which he believes originates in a parallel universe to his own and enters a person's body via their dæmon. Fearing the effects of Dust, the Magisterium conducts experiments on children in order to find a means of inoculating them against it.
After Asriel departs, the college is visited by Mrs. Coulter, who offers to take Lyra north as her assistant. Lyra is entrusted with an alethiometer by the master of the college. The alethiometer is a device that is able to reveal the answer to any question asked of it by a trained user, and is the last in existence since the Magisterium forbade their use. While unable to operate the device, Lyra accepts the gift and takes it with her to Mrs. Coulter's home. When Mrs. Coulter arouses Lyra's suspicions by delaying their journey, Lyra discovers that Mrs. Coulter is head of the General Oblation Board—the "Gobblers" who have been kidnapping local children. She also discovers that her best friend Roger and her Gyptian friend Billy Costa have been taken by the Gobblers to the north.