A sudden outbreak of St. John's Dance causes people in the streets of Aachen, Germany, to experience hallucinations and begin to jump and twitch uncontrollably until they collapse from exhaustion.

Dancing mania was a phenomenon that may have had biological causes, which occurred primarily in mainland Europe between the 14th and 17th centuries. It involved groups of people dancing erratically, sometimes thousands at a time. The mania affected adults and children who danced until, allegedly, they collapsed from exhaustion and injuries, and sometimes died. One of the first major outbreaks was in Aachen, in the Holy Roman Empire, in 1374, and it quickly spread throughout Europe; one particularly notable outbreak occurred in Strasbourg in 1518 in Alsace, also in the Holy Roman Empire.

Aachen is the 13th-largest city in North Rhine-Westphalia and the 27th-largest city of Germany, with around 261,000 inhabitants.

A hallucination is a perception in the absence of an external stimulus that has the compelling sense of reality. They are distinguishable from several related phenomena, such as dreaming, which does not involve wakefulness; pseudohallucination, which does not mimic real perception, and is accurately perceived as unreal; illusion, which involves distorted or misinterpreted real perception; and mental imagery, which does not mimic real perception, and is under voluntary control. Hallucinations also differ from "delusional perceptions", in which a correctly sensed and interpreted stimulus is given some additional significance.