From the introduction:
Haddonfield, Illinois. A small suburban town where, fifteen years ago, a grisly murder took place. A young boy no older than six named Michael Myers murdered his sister, and was then committed to the Smith’s Grove Sanitarium. Michael is put under the care of Dr. Loomis, a child psychiatrist. Michael’s nearly catatonic state and lack of response to treatment, combined with his total lack of remorse for the murder of his sister, causes Loomis to call him a beast and “evil incarnate”. After fifteen years in the asylum an unknown incident allows the now adult Myers to escape. He hijacks a car and begins the drive back to his hometown, where he would start his first killing spree. From here, things get a little complicated. There are five different timelines to choose from. The first two split split off from the first movie, and both feature the complete aftermath of Michael’s initial rampage (Halloween 1 and 2).