After Tex Murphy was injected with a Trance Inducer in 2050, he experienced a memory loss. To recover his memories he needs a Trance Interrupter, as well as a Trance Key. Since Margaret Leonard is the only one who knows the key, and she is dead, Tex is unable to find his own key.
In Tex Murphy and the Tesla Effect Tex is able to discover his own key using several clues laid out by Margaret. The key is revealed to be the phrase sleep no more.