edited the summary of Needful Things Sunday, September 19, 2010.
A new shop named "Needful Things" opens in the town of Castle Rock, Maine, sparking the curiosity of its citizens. The proprietor, Leland Gaunt, is a charming elderly man who always seems to have an item in stock that is perfectly suited to any customer who comes through his door. The prices are surprisingly low, considering the merchandise - such as a rare Sandy Koufax baseball card, a carnival glass lampshade, and a splinter believed to be from Noah's Ark - but he expects each customer to also play a little prank on someone else in Castle Rock. Gaunt knows about the long-standing private grudges, arguments, and feuds between the various townspeople, and the pranks are his means of forcing them to escalate until the whole town is eventually caught up in madness and violence.
Sheriff Alan Pangborn becomes wary of Gaunt as soon as the shop opens. However, his lover, Polly Chalmers, dismisses his suspicions and buys an ancient charm that relieves the arthritis pain in her hands. The tension between them grows after Nettie Cobb, Polly's housekeeper, and her enemy Wilma Jerzyck kill each other in a confrontation sparked by pranks played on them by others.
Many other rivalries begin to fester, spurred by the personal motives of the people involved (drugs, secret pedophilia, bad business dealings, religious disagreements, etc.). Gaunt eventually hires petty criminal John "Ace" Merrill as his assistant, holding out the promise of buried treasure that could relieve the debt he owes to a drug dealer. Ace's first assignment is to retrieve a crate of firearms, which Gaunt begins to sell to his customers so they can protect their property. It is revealed that Gaunt has traveled the world for centuries, selling useless junk that appears to be whatever the buyer desires most. They become so paranoid about keeping the items safe that they eagerly buy up the weapons that he inevitably offers for sale and trade away their souls.
With the violence in Castle Rock rapidly escalating, Ace and selectman Danforth "Buster" Keeton (who has embezzled thousands of dollars in public funds) plant dynamite all over town. Alan sets out to kill Ace, wrongly believing him to be responsible for a car accident that killed Alan's wife and son, and Polly realizes the evil of the charm she bought and destroys it. As the dynamite bombs explode and Keeton is killed, Ace takes Polly hostage and demands that Alan tell him where the treasure is hidden, only to be shot dead by one of Alan's deputies. Alan then faces off against Gaunt, steals a valise that contains the souls of all his customers, and forces him to leave town. As Gaunt departs, he transforms into a demonic dwarf, and his antique car becomes a buckboard decorated with a sign that no one paid attention to in the shop: "Caveat emptor" ("let the buyer beware"). The night's madness begins to subside, and the survivors ponder an uncertain future in what remains of Castle Rock.
The novel ends as it begins, with a first-person narrative indicating that a new and mysterious shop is about to open in a small Iowa town - an implication that Gaunt is about to begin his business cycle all over again.