The story starts when Lyra Belacqua— a supposedly orphaned, 12 year old girl residing at Jordan College, Oxford—secretly enters and hides in the forbidden 'Retiring Room' in the college, despite resistance from her dæmon, Pantalaimon — an animal-formed, shape-shifting manifestation of her soul. Hidden behind an armchair, Lyra and "Pan" sees the Master of the college putting poison into wine intended for the visiting Lord Asriel, Lyra's uncle, in an attempt to assassinate him. Lord Asriel later enters after the Master of the college has left and Lyra, having by now hidden in a wardrobe, bursts out and immediately warns him that the wine is poisoned. Rather than punishing her for being where she should not, he allows her to stay hidden if she will spy on the other attendees at his upcoming meeting. When the meeting commences, Lord Asriel shows the resident scholars and the master pictures of the Aurora Borealis (the 'Northern Lights' of the title) and the mysterious elementary particles called Dust. Shortly after, Lord Asriel travels to the Arctic North, and Lyra continues her studies at the college.
After the meeting the master and the librarian discuss Lord Asriel's journey to the north and allude to another, invisible and untouchable, world.
When “the Gobblers,” who have become a recent urban legend, kidnap her friend Roger a kitchen boy from the college, Lyra vows to rescue him. But instead, an important visitor, a woman named Mrs. Marisa Coulter, offers to take Lyra away from Jordan College to become her apprentice. Lyra assents, but before she leaves, the Master of the college entrusts to her (with the condition that she keep it absolutely secret) a priceless object previously given to the College by Lord Asriel: an alethiometer. Resembling a golden, many-handed pocket-watch, it can answer any question asked by a skilled user. Although initially unable to read or understand its complex symbols, Lyra takes it with her to Mrs. Coulter's flat. Soon after, Lyra becomes suspicious of Mrs. Coulter's motives when her dæmon (a golden monkey named Ozymandias) searches Lyra's room for the alethiometer.
At a cocktail party hosted by Mrs. Coulter, Lyra discovers that Mrs. Coulter heads an organization known as the "General Oblation Board" and that this board is in fact, the "Gobblers" who have been kidnapping children. Horrified, Lyra flees Mrs. Coulter's flat during the party.
While being pursued by the "Gobblers", Lyra is rescued by the Gyptians, nomadic, canal-boat-dwelling people, who afterwards reveal that Lord Asriel and Mrs. Coulter are none other than Lyra's father and mother. She also learns that many children like Roger have been disappearing from among the Gyptians, and that the Gyptians are planning an expedition to the north to rescue them. During her time with the Gyptians, Lyra intuitively begins to learn how to operate the alethiometer.
En route, Lyra enlists the help of a sentient, armoured bear called Iorek Byrnison. Iorek is an exiled bear prince who is a perfect example of his culture.
Lyra is captured by a party of hunters who take her to an experimentation facility in Bolvangar, where she discovers that the children are being subjected to an experimental process called "intercision." This process severs the tie that binds them to their daemons, effectively removing their soul. Inside, she locates Roger and devises an escape plan. She is caught spying and narrowly escapes suffering the intercision process herself; she is rescued, unfortunately, by Mrs. Coulter, who tries to take the alethiometer. Narrowly escaping her clutches once again, Lyra leads the other children from the facility and is rescued by Lee Scoresby in his hydrogen balloon. With them are Roger and Iorek Byrnison.
Having found Roger, Lyra now determines to deliver the alethiometer to Lord Asriel, believing that he needs it for his purposes. He is imprisoned at Svalbard, the armoured bears' fortress, because the church opposes his experiments on Dust. As they travel to Svalbard, bat-like cliff-ghasts attack the balloon; Lyra is thrown out but lands safely, only to be captured by the armoured bears. She tricks the usurping bear-king, Iofur Raknison, into fighting Iorek Byrnison, who regains his throne. Thereafter, she travels to Lord Asriel’s cabin, accompanied by Iorek and Roger.
Despite being imprisoned, Lord Asriel has become so influential that he has accumulated the necessary equipment to continue his experiments on Dust. After explaining to Lyra the nature of Dust, an emanation from another world, and the existence of parallel universes, he departs, taking Roger and much scientific equipment. Lyra pursues them, having discovered that she has indeed brought her father what he wanted, though not in the way she thought. It was not the alethiometer he needed, but Roger: the severing of the child-daemon tie releases an enormous amount of energy, which Lord Asriel needs to complete his task. Roger dies when Lord Asriel separates him from his dæmon, and with the enormous energy released -- combined with his specialized equipment -- Lord Asriel is able to tear a hole through the sky into a parallel world. Lord Asriel walks through into the new world. On Pantalaimon's advice, Lyra follows. This concludes the first novel, with the trilogy continuing in the next book, The Subtle Knife.
source: Wikipedia
