Much of Robertson's "realism" however was borrowed from the novelist Charles Dickens. The heroine of Caste is named Esther, and like Dickens's Esther Summerson of Bleak House, she falls in love with a man above her station. Like another Dickensian heroine, Little Dorrit, she struggles to support her family, which includes a worthless father and a sister in the theatrical profession.
The second act of the play exposes the ridiculousness of English class conventions. After Esther has married her sweetheart George in secret, George's mother the Marchioness comes for a visit. Esther hides out of sight, but then faints when she overhears the Marchioness denounce marriage between her son and a "plebian."
George is in the army and must travel to India due to the Sepoy Rebellion (historically, an event that began ten years before the premiere of the play). The India connection is what provokes comparisons between the English class system and the caste structure of South Asia. George and Esther bristle under these restrictions, daring to love in spite of social prejudice.
However, Robertson does not attack the class system so much as suggest it should be more flexible. In the third act, George declares:
Oh, caste's all right. Caste is a good thing if it's not carried too far. It shuts the door on the pretentious and the vulgar; but it should open the door very wide for exceptional merit. Let brains break through its barriers, and what brains can break through love may leap over.
Though theatre historians like to hold up Robertson as a precursor to George Bernard Shaw and the New Drama of the 1890s, it's important not to overstate the case. For the most part, Robertson is a conventional though talented playwright. Caste broke few barriers in 1867. It would be plays like Mrs. Warren's Profession that would bring real change a few decades later.