It is based on 1 Samuel 3.  In this episode, Samuel begins as an apprentice priest and ends up as a prophet recognized by all Israel.  In the story itself, God calls out to Samuel four times.  The first two times, neither Samuel nor Eli have a clue about who is speaking.  The third time, though, Eli perceives that the voice belongs to God.  When God calls to Samuel for a fourth time, the deity announces judgment on Eli’s priestly house, primarily due to the egregious behavior of his two sons.  Samuel has to deliver this bad news to Eli.  The episode concludes by letting us know that God has replace a dominant priestly ministry with a dominant prophetic ministry.