In France, the bachelor Frederick St. John seeks a reclusive existence in order to restore his finances; unexpectedly, he finds himself interposing in the engagement of Adeline de Castella to a French baron, with tragic consequences.
Prompted by concerns that Charlotte's envy of Benja (who will inherit Alnwick in place of her son) might lead her to harm him, his relative Isaac St. John is left as a guardian to the child by a provision of the will.
Eventually judged to be an accident, nonetheless Charlotte is deeply disturbed by the event: she becomes prone to hallucinations about the child and the lighted toy, and decides to leave Alnwick with her son George.
Her close companions, Rose Darling (Charlotte's sister) and Mary Carr, finally manage to contact Frederick and take him to see Adeline, whose body is preserved as part of its reception before the funeral.
Frederick suspects her of hereditary madness and that she intends to marry Isaac in order to acquire his fortune; various incidents suggest to him that her jealousy of him might lead her to commit violence against the other women also there.
[2]: 94–95 The newspaper The Spectator published an article entitled 'Madness in Novels' (1866) which compared St. Martin's Eve with Mary Elizabeth Braddon's Lady Audley's Secret (1862) and the anonymous The Clyffords of Clyffe.