Answer: The locked door of the secret garden.
Mr. Craven is the master of Misselthwaite Manor. He suffers from ill health, but spends most of his time abroad. He is still mourning the death of his wife, ten years before the novel begins. Because of his pain, he does not want to spend time in his house or with Colin (his son). His heart is closed. His closed heart is symbolized by the closed door of the secret garden.
