Beneath the streets of New York City, a series of disappearances has begun to draw attention. George Cooper, a once-renowned photographer turned down on his luck, takes interest in the homeless population, which seems to be the primary target of the unsolved vanishings. Simultaneously, A.J. Shepherd, the head of a local soup kitchen, notices that his regulars are also diminishing in numbers inexplicably. Both men are independently drawn into the mystery, driven by a desire to uncover the truth behind the unsettling occurrences plaguing the city's underbelly.

Their investigation converges as they discover an alarming secret lurking in the city's sewers. Toxic waste has given rise to a group of mutated, cannibalistic creatures known as "C.H.U.D.," an acronym denoting "Cannibalistic Humanoid Underground Dwellers." As George and A.J. delve deeper into this subterranean world, they join forces with Captain Bosch, a police officer whose wife is among the missing. The trio faces bureaucratic resistance and disbelief from authorities, who seem to have a vested interest in keeping the existence of these beings hidden from the public. Confronted with a growing threat from below and a conspiracy of silence from above, they must expose the truth before more lives are lost to the darkness beneath the city.