A mentally handicapped boy, Howie, is the only person in town not affected by ancient parasites unearthed in the local mine. The parasite removes all inhibitions from those it infects. As the town becomes a chaotic and dangerous place, Howie must find a cure and save the townspeople.

A mentally challenged boy named Howie (Neil Patrick Harris) is the last unaffected person in a small town overrun by a strange madness. Miners unearth ancient parasites, in the shape of worms, that attack the brains of their hosts, entering through the nose or ears. While the infected townsfolk lose all their inhibitions, Howie must save his sister Sheila, the only person who truly cares for him. Deprived of Sheila's guidance for the first time in his life, Howie struggles to evade his maddened neighbors and destroy the parasites. Some of the infected break into the sibling's home and infect Sheila, but Howie's infection fails, the parasite leaving his body only moments later.

Howie experiments with his former parasite in a jar. Its form reminding him of a slug, he tries to kill it with salt, but it only revives it. Instead, sunlight through a nearby window then bathes the jar with light and kills it. At their home, Howie ties up Sheila outside until daylight. Sheila is exposed to sunlight and her parasite flees her body, returning her to normal. She reassures her brother that the hurtful things she had said to him was the work of the parasite; she would never willingly say such things. Sheila realizes that due to his mental illness, he lacks what the parasites need in a host and so are not able to infect him.

After Sheila finds out about how Howie's parasite revived, she concludes that the parasites must have their hosts eat large quantities of salt in order to survive, which leads them to investigate the local salt mines for the possibility that that is where they are during the day.

Sheila is eventually re-infected. At first he is unsure whether it's her or not, until her frustrated parasite screams insults at him. Howie and Sheila's father had taught him how to safely set up dynamite, which he does at the entrance of the mine.

He waits until just before sunrise, when all the infected are headed to the mine to escape the daylight. Just before they reach the darkness of the mine, Howie detonates the explosives, blocking the entrance. With seconds until sunrise and the mine blocked, the parasites are completely exposed with nowhere to go. The parasites flee their hosts, leaving them unharmed, to die on the ground. The three original miners that became carriers and distributors of the parasites unfortunately die from the multiple exit wounds the many escaping parasites create. In the end Howie is thanked as a hero, his "weak" brain having been the strength that saved him and his neighbors.

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Wikipedia