Halloween was always Tim's favorite holiday-until his mother was diagnosed with brain cancer. Now, all he wants is a quiet night in. That is quickly disrupted when his friends pay him an unwanted visit, after stealing a mysterious sack from the doorstep of a local crazy. This sacrilegious act unleashes the Aos Si, a Celtic spirit that walks the Earth on Halloween. They must band together and survive the night or make the ultimate offering-their lives-this Halloween, all who trick are treats.