Buffy has been described many times as someone who 'shot first ask questions never'. So when a man with eyes that seem to glow suddenly appear in the school's library, Buffy attacks and severely injuries the man. Xander and Giles are able to stop Buffy from killing the man.

Left with an injured and unconscious man and a Slayer who wants nothing more than to kill him, Xander offers to take the man and care for him until they can figure out who and what he is and more importantly what he wants with the Hellmouth.

Willow points out that if the stranger is dangerous Xander won't be able to stop him if he's a bad guy. In order to bring order back to his library and to keep Buffy from killing someone who appears to be more or less human Giles offers to cast a spell on Xander that will incapacitate the man if he tries to harm Xander by physical or magical means.