In HBO’s spirited fantasy epic “His Dark Materials,” Lyra Belacqua (Dafne Keen ), a runaway girl with a secret destiny, comes into possession of an alethiometer , a golden clockwork gizmo with the power to answer any question. One mystery, however, still eludes her.

“I don’t think I understand any grown-ups at all,” she says.

“His Dark Materials,” beginning Monday and based on the religiously skeptical trilogy by Philip Pullman , is a story about witches and giant polar bears, magic (or quasi-magic) dust and actual spirit animals. But above all, it is a story about parallel worlds, alike and yet wildly different, separated by an imperceptible barrier: the worlds of childhood and adulthood.

The story opens at Oxford, but not our Oxford. In the “Materials” world — a kind of steampunk mélange both more and less technologically advanced than our own — the cathedral-like university is both hugely powerful and closely overseen by the Magisterium, a repressive theocracy with not a few similarities to the Roman Catholic Church.

Lyra, raised as a foundling by the scholars, has the run of the school, blissfully ignorant of the political-religious infighting around her, until the man she knows as her uncle Asriel (James McAvoy ), turns up claiming the stunning — and heretical — discovery of another universe. The ensuing scandal, and a conspiracy involving the abduction of children, sends Lyra on a hero’s quest to the frozen north, chased by the blasphemy police.