Maria Vittoria, foster-daughter of an influential Roman family, has fallen in love with her German tutor, Daniel, recently made a bishop of the Catholic Church. Maria's foster-father wants to marry her off to a German merchant banker, but the headstrong Maria has other plans.

After causing a scandal at the Vatican the lovers are obliged to leave Rome. Daniel takes Maria back to his family home in Heidelberg where she meets Anna Regina, his possessive elder sister. The illicit love affair conflicts with Anna's ambitions for her brother's career, and when Daniel openly declares his commitment to Maria, Anna has to take action. Daniel mysteriously falls ill on the following day and remains in a deep sleep.

Anna Regina then accuses Maria of having cast a spell on Daniel. The townspeople turn on Maria and she is imprisoned for witchcraft. A few weeks later, Cardinal Gonzaga, the Inquisitor, arrives from Rome to cure the Bishop and prosecute his mistress. Daniel recovers and for a while accepts Anna Regina's allegations against the woman he loves. Meanwhile, Maria is tortured but insists on her innocence; without a confession she cannot be sentenced.

Maria manages to escape from jail, but instead of fleeing Heidelberg to save her life, she seeks out Daniel. Reunited in the church they have a brief moment of happiness together.

In a frenzy of witch-fever the townspeople are searching the streets for Maria. Soon Gonzaga arrives with soldiers. He announces that Maria, by escaping, has confirmed her guilt and must die.

The pyre is being prepared outside...