Complex sentence: She explains Stevens is in the "Source Code", an experimental device created by Dr. Rutledge (Jeffrey Wright), scientist for Beleaguered Castle, that allows its user to experience the last eight minutes of another compatible person's life within an alternate timeline.
Split into simpler sentences: She explains Stevens is in the "Source Code", an experimental device created by Dr. Rutledge (Jeffrey Wright), scientist. The Code allows its user to experience the last eight minutes of another compatible person's life within an alternate timeline.