When Alex Easton, a retired soldier, receives word that their childhood friend Madeline Usher is dying, they race to the ancestral home of the Ushers in the remote countryside of Ruritania.

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What they find there is a nightmare of fungal growths and possessed wildlife, surrounding a dark, pulsing lake. Madeline sleepwalks and speaks in strange voices at night, and her brother Roderick is consumed with a mysterious malady of the nerves.

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Aided by a redoubtable British mycologist and a baffled American doctor, Alex must unravel the secret of the House of Usher before it consumes them all.