The Burning Secret by Stefan Zweig

A charming stranger arrives at a summer resort and insinuates himself into the life of a widowed mother and her lonely young son, winning the boy’s naïve admiration by confiding a supposed secret love for the mother and enlisting the child to help conceal their intimacy; as the boy slowly perceives he has been manipulated, his hurt and childish act of exposing the truth destroys the illicit liaison, humiliates the adults, and precipitates a painful loss of innocence and moral awakening for the child.

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