Something ghastly is afoot in Victorian Yorkshire. Something that kills. Bodies are washing up in the canal, their skin a waxy, glowing red, but just what is this crimson horror?

Madam Vastra, Jenny and Strax are despatched to investigate the mystery. Strangely reluctant to assist their enquiries is Mrs Winifred Gillyflower, matriarch of 'Sweetville', a seemingly utopian workers' community.