Late one night in New York City, Flora Bosch walks her dog on a deserted downtown street. Suddenly, a manhole cover opens and a creature pulls the woman and her dog into the sewer. The following day, former fashion photographer George Cooper and his girl friend, Lauren Daniels, a model, settle into a new apartment. Meanwhile, at the local police precinct, New York Police Department (N.Y.P.D.) Captain Bosch is ordered by Chief O’Brien to remain silent about a recent increase in missing persons. Afterwards, Bosch tells a reporter named Murphy that there is nothing going on. Later, Captain Bosch investigates a missing persons report filed by the “Reverend” A.J. Shepard, who runs a soup kitchen. For two weeks, Shepard has not seen ten or twelve of his regulars, street people who live underground. George Cooper, who has been documenting the plight of homeless people for a magazine, is called to the precinct by Doris Monroe, one of his subjects. When they leave the police station, a detective named Jackson follows them into a subway station. They disappear into a tunnel and Jackson reports to Bosch that he lost them. Doris leads George to a remote area where she lives with her brother, Victor, and his friend, Hugo. The police arrested Doris because she tried to steal an officer’s gun, which Victor claims he needs to protect them from something that attacked him and left a large, festering wound on his leg. Bosch returns to the soup kitchen and asks Reverend Shepard to identify the people in George’s published photographs, but he is suspicious of a police captain so interested in missing homeless people. Bosch admits that there are others missing, including his wife, Flora. Reverend Shepard identifies Doris, Victor, and Hugo, and then requests Bosch reciprocate by telling him about an ongoing Environmental Protection Agency (E.P.A.) probe. Bosch responds that it is an annual inspection of the subways and sewers, but Reverend Shepard insists that this year’s probe is different, lasting four weeks instead of the usual one. He shows the captain a Nuclear Regulatory Commission (N.R.C.) Geiger counter he found below ground, leads Bosch to an area where his regulars once lived, and demonstrates that there are heightened levels of radiation. Meanwhile, outside George and Lauren’s apartment, a creature climbs out of a manhole and attacks a man, leaving his traumatized granddaughter, Cindy, behind. In the morning, the little girl tells police a monster came out of the sewer and ate her grandfather. The officer does not believe her, but Bosch orders Jackson to place two officers on every street corner and four in each subway station, and puts out an all-points-bulletin on his missing wife, Flora. Bosch then telephones Chief O’Brien and demands a meeting with the Police Commissioner and a representative of the N.R.C. Bosch and Reverend Shepard go to George’s apartment, but he is not home. However, they discover the photographs of Victor’s wounds. When they meet with the Commissioner, Chief O’Brien, and Mr. Wilson of the N.R.C., Bosch explains that he can no longer ignore the surge in missing persons, and shares Reverend Shepard’s evidence, but Wilson claims that it means nothing. Reverend Shepard shows them George’s photographs and threatens to go the newspapers. Meanwhile, Murphy, the reporter, approaches George and informs him that police followed him and Doris the day before. He wants to know more about the underground people, but George refuses to cooperate. Back at the meeting, the Commissioner confides that the E.P.A. discovered slightly elevated radiation levels and Wilson explains that the N.R.C. was prevented by court order from transporting hazardous waste, so it remains in the city. However, he claims there is no connection to the missing people and dares Reverend Shepard to go to the press. After Shepard leaves, Wilson receives a telephone call and reveals to the others that two gas company employees had found the creature, a “Cannibalistic Humanoid Underground Dweller” (C.H.U.D.), dead. Later, Wilson, Bosch, the Commissioner, and O’Brien visit the location where the creature was found and learn from the coroner that it was once human and died from asphyxiation resulting from a gas leak. Bosch wonders if there could be others. Wilson orders a team to go underground and commands Bosch not to send any police down because he does not want any publicity. Bosch, however, ignores the order and sends a team with flamethrowers to accompany Wilson’s investigators. Bosch and Wilson supervise the operation via video monitors and witness as their men encounter a C.H.U.D. and the video feed goes dead. Later, the Commissioner and O’Brien attempt to convince Wilson that the public needs to be alerted, but he argues that any remaining creatures can be killed using gas or other means. Back at his apartment, George worries that the reporter, Murphy, might be on to something and realizes his photographs of Victor are missing. He telephones Murphy and suggests they meet in a subway station. George accuses Murphy of stealing the photographs, but the reporter insists the police have them and asks George to take him to the tunnel where the photographs were taken. Meanwhile, Bosch updates Reverend Shepard about the C.H.U.D. creatures and warns him that Wilson’s plan might drive them above ground. Shepard then goes underground to urge a homeless man named Val to come up, but one of Wilson’s henchmen kills Val and locks Shepard in the sewer. Looking for a way out, Reverend Shepard goes deeper into the underground and discovers a colony of humanoids. Elsewhere in the tunnels, a creature attacks Murphy, but George escapes. Back at the apartment, Lauren goes to the basement and finds a door in the floor leading underground. She drops a photo album, goes down to retrieve it, and finds the body of a mutilated dog. After calling the police, she takes a shower and blood spurts from the drain. Elsewhere, Bosch and two officers respond to a call from the docks and identify a corpse as the captain’s wife, Flora. Nearby, a group of humanoids attack the inhabitants of a diner leaving no survivors. Police, government officials, and media swarm the scene, and Wilson orders his men to seal every manhole in a ten-block radius, shut down the subway and turn off the power lines. Underground, George returns to the homeless encampment and shoots the wounded Victor, after he tries to eat him. Doris also attacks George, but Reverend Shepard arrives and saves him. Bosch goes to the diner and confronts Wilson, who orders gas pumped into the underground. Lauren attempts to leave the apartment, but comes face-to-face with a humanoid on the stairs. She locks herself in and barricades the door. Two police officers responding to Lauren’s call are killed by the humanoid as they attempt to enter the building. Meanwhile, George and Reverend Shepard frantically search for a way out of the sewers, but find the manholes blocked. They discover an enormous stockpile of toxic waste the government has been dumping for years. The containers are marked “C.H.U.D.” They find the bodies of the N.R.C. investigation team and use the video camera to reach the N.R.C. control room and contact Bosch. Bosch directs them to meet him at a nearby manhole, and Reverend Shepard tells him about Wilson’s cover-up. Meanwhile, the humanoid enters Lauren's apartment. She decapitates it using a samurai sword, then flees in an abandoned police cruiser. Wilson tries to prevent Bosch from rescuing Reverend Shepard and George, and the police captain informs Wilson that he knows what C.H.U.D. actually stands for. Bosch slugs Wilson and moves the truck blocking the manhole. When Bosch attempts to lift the cover, Wilson shoots him. George and Reverend Shepard climb out and Wilson attempts to run them over with the truck, but Lauren arrives in time to warn them and they leap to safety. Reverend Shepard uses Bosch’s gun to shoot Wilson. The truck comes to rest with one tire in the manhole, and bursts into flames. A wounded Bosch compliments Shepard on his shot as Lauren and George embrace.