What Moves The Dead by T. Kingfisher

Here is a summary with spoilers for What Moves The Dead

Lieutenant Alex Easton and kan horse, Hob, travel from Gallacia to Ruravia to visit kan ailing twin friends, Madeline and Roderick Usher. Upon arrival, Easton meets a frail Roderick, who questions why Alex has come; and Alex says it’s because of a letter he recieved from Madeline. Inside, Easton finds Madeline even weaker than her brother and meets James Denton, an American doctor who has been unable to determine the cause of Madeline’s decline. Denton has urged the Ushers to leave the house for her health, but Roderick refuses, fearing the move would kill her. In an aside, Easton reflects on Gallacian pronouns, explaining the ka/kan system adopted for sworn soldiers like ka, a holdover from ka's military service during a time when women fought to remain in the army.

At breakfast, Easton offers to help by hunting for food, and Denton shares his belief that Madeline suffers from catalepsy and anemia, though these are symptoms rather than a true diagnosis. While wandering the house, Easton finds Madeline gazing at the lake; she confesses that she feels her end drawing near. After helping her to bed, Easton discovers that kan loyal batman, Angus, has arrived. Angus, deeply superstitious, senses evil in the Usher house and reports that local villagers share this belief, speaking of witch-hares—supernatural hares unafraid of humans—that haunt the area. Though Angus refuses to hunt on the property, he agrees to fish, wary of the malevolent forces he believes are at work.

Easton was planning on hunting red meat to help Madeline’s anemia, he ends up spotting Maeline heading toward the lake. 

That night, Easton wakes to the sound of someone moving through the hall. Though ka follows the figure, kan tinnitus flares up, forcing ka to return to bed. The next day, Easton and Denton ride out and meet Miss Potter, someoen who as been helping Easton learn about the strange fungi that grow in the area. As Denton examines the redgills, Easton notices another bizarre hare that drags itself away like a climber on flat ground. At dinner, Angus reports catching fish filled with slimy felt, likely infected with fungus. That night, Easton again hears movement and finds Madeline wandering the hall, seemingly sleepwalking. When Easton tries to guide her back to bed, Madeline becomes briefly coherent but dismisses the concern. Unable to sleep afterward, Easton watches eerie, flashing lights in the tarn, reminding ka of bioluminescent plankton, though the lights’ deliberate pattern is unnerving.

The next night, Easton and Denton meet in the hall and talk on the balcony. Denton recounts a nightmare about severed limbs moving and agrees that the tarn is dismal, comparing it to deadly but colorful streams in the US. Easton describes the flashing lights ka saw, and Denton reveals that Madeline nearly drowned in the tarn months ago during a cataleptic fit. They briefly glimpse more lights before returning to their rooms.

The next morning, Easton hears Madeline again and follows. Her stiff, awkward walk unnerves ka, resembling the movements of the strange hare seen in the forest. When Easton tries to rouse her, Madeline speaks in strained, incoherent words, mentioning “too many” and struggling to breathe. Her unsettling smile and the white hair left on Easton’s hand terrify ka. Escorting her back to bed, Easton is troubled to find her maid absent and realizes just how inhuman Madeline’s movements have become.

Easton speaks with Roderick about Madeline, learning that her maid died by suicide three months prior. Roderick admits he’s considered burning the house down, though Denton deflects by suggesting Easton merely meant Roderick should hire another doctor. Denton, though unable to fully diagnose Madeline, notes her bizarre symptoms—such as rapidly regrowing hair—and Roderick refuses to bring in outside help. While trying to research hares in the Usher library, Easton finds the books ruined by mold and mildew. At this point Roderick is certain the house will kill both himself and Madeline. Denton theorizes that an illness like rabies could affect both hares and humans, and Easton learns the maid’s death wasn’t due to disease. When Easton later encounters a trio of unsettling hares—one of which he shoots—it reanimates and eerily stares at him, confirming something unnatural is at work.

Though Denton tries to rationalize the event, Easton remains disturbed. The next morning Easton wakes to the news that Madeline has died of a cataleptic fit. As time passes, Easton begins to suspect that Madeline may not be truly dead. He returns to the crypt and finds her body with a broken neck. Convinced she was murdered, Easton suspects either Denton or Roderick. Meanwhile, strange fungal growths, unexplained hares, and the lack of rats in the house deepen the mystery. A conversation with Miss Potter reveals that certain fungi can hunt microscopic organisms using thread-like hyphae resembling white hairs—like those growing on Madeline.

Easton brings Miss Potter to examine Madeline’s body, but they discover it missing. White hairs remain on her shroud, and Miss Potter confirms they are fungal hyphae. Alarmed, the group suspects the fungus may be infecting people as well. They retrieve a dead hare and perform an autopsy at the dinner table, discovering its lungs are packed with fungus. To their horror, the hare reanimates—its head even moves independently after decapitation. The fungus, they realize, is controlling the hare's nervous system. With the lake likely contaminated, they burn the hare and the tools. That night, they spot glowing water and a hare watching them. Easton sends Angus and Miss Potter to get help while he and Denton remain behind.

Easton confronts Denton about Madeline’s broken neck. Denton reveals that Madeline began acting strangely after nearly drowning in the lake—sometimes lucid, sometimes disturbingly different. After assuming she was dead, Roderick had her moved to the crypt, only for her to awaken with a new voice. Denton now believes the fungus infected both Madeline and her maid. Roderick, trying to destroy the fungus’s hold on Madeline, broke her neck. When Easton and Denton return to the crypt to burn her body, they find it gone—and two sets of footprints confirm Madeline walked out herself.

They rush upstairs to find her in Roderick’s bed, her body visibly decaying but animated. She reveals that the tarn—the lake—has been infecting and learning from its hosts, growing more intelligent over time. The fungus has kept her alive, but it’s now overtaking her completely. Madeline pleads for Easton to help teach the tarn, claiming it has grown and won’t harm its next host. Horrified, Easton and Denton trap her in the room. Roderick, realizing what he must do, sends away the remaining servants and sets fire to the house of Usher.

Denton and Easton watch the house burn for two days, preventing anyone from intervening. Afterwards, they wonder if they too are infected and debate how to stop the fungus entirely. Angus and Miss Potter return with sulfur, which can kill fungi. They travel to the tarn and dump in 1,200 pounds of sulfur. The lake glows, dims, and finally goes dark. As a final precaution, Aaron vows to monitor the tarn and burn any animals that approach it, while Easton reflects on the lingering danger—and the cost of what was lost to the fungus that once lived in the lake.