Mysteries, as their very name indicates, are stories about things beyond ordinary human ken. This is a genre, after all, invented by Edgar Allan Poe, for whom the boundaries between his “tales of terror” and his “tales of ratiocination” were porous. Even Sherlock Holmes, that human apotheosis of logic, gets thoroughly spooked by the Great Grimpen Mire in “The Hound of the Baskervilles.”

Chief Inspector Armand Gamache, the character at the center of Louise Penny’s beloved and successful series, is an avowed believer in the power of things seen and unseen. Alert to the atmosphere of place and the saving grace of poetry, Gamache also trusts his gut when it comes to the presence of more malevolent forces. “A World of Curiosities,” the 18th Gamache novel, begins with our beloved detective conversing with the dead and ends with a bonfire and talk of witches. The eeriest Gamache novel yet, “A World of Curiosities” is also one of Penny’s most intricately plotted and harrowing.

Something wicked this way comes to Gamache at home, in his beloved village of Three Pines, a place not found on any maps. Gamache’s refuge is breeched when the villagers host a celebration for two young women who have graduated from engineering school at the University of Montreal. One of the graduates is Harriet Landers, the niece of Myrna, the local bookshop owner; the other, a woman named Fiona Arsenault. Gamache encountered Fiona years ago on his first case — a murder investigation — that he and his now son-in-law, Jean-Guy Beauvoir, worked on together. Fiona, then an adolescent, was found guilty of the murder of her mother and served time in prison. Gamache, however, had his doubts about that conviction and championed Fiona’s right to an education while incarcerated.

About Fiona’s younger brother Sam, however, Gamache has never had any doubts: His sixth sense has always told him that handsome Sam (whom everyone else finds charming) is malicious, maybe worse. Unfortunately, Gamache cannot prevent Sam from joining his older sister at the graduation festivities in Three Pines and subsequently getting comfy at the local inn.

Myrna also finds her personal space infiltrated by a malign presence. Her home, a cozy loft above the bookshop, has begun to feel cramped now that her boyfriend, Billy, is living with her and Harriet is camping out on her living room sofa. Coincidentally (or not) Billy has recently received an odd letter dated 1862, written by his great-great-grandfather, a stonemason. The letter recounts how Billy’s great-grandfather was hired by an anonymous employer to brick up a room in the dead of night in the same building that now houses Myrna’s loft and bookshop. When the letter is passed around to Gamache and his Three Pines friends, Myrna quietly comments, “‘The Cask of Amontillado,’ ... saying what they were all thinking.” But the “world of curiosities” that’s found sealed inside that hidden room next to Myrna’s loft is more menacing than what’s entombed in Poe’s infamous wine cellar.

That’s but a sampling of the strange goings-on in “A World of Curiosities,” where even secondary plots are tinged with a supernatural aura. (Reine-Marie, Gamache’s beloved librarian wife, for instance, is searching for a “grimoire” — “a book to summon demons,” once owned by a 17th-century female mystic.)

And yet, there’s another all-too-real dimension to this story. In her acknowledgments, Penny references the tragedy that inspired the characters of Harriet and Fiona: the mass murder at the Polytechnic University in Quebec on Dec. 6, 1989, of 14 female engineering students and the wounding of 13 others. Anyone who’s read Penny’s novels knows that as entertaining as they are, they are also charged inquiries into the actual evils that human beings and societies do.

The misogyny that fueled that real-life massacre is also a factor in some of the more supernatural elements in “A World of Curiosities.” Only a mystery writer of great stylistic range and moral depth could handle the demands of such a shifting — and potentially sensitive — story as this one. Fortunately, as she proves once again, Penny is all that and more.