Substance is not just a horror with elements of surrealism, but also a metaphorical parable. The film raises an important theme of acceptance of one's age and inner fears of aging that many people, especially women, face. Beauty standards imposed by society demand youthfulness and impeccable appearance, especially for those who work in public spheres. This social "contract" between youth and success is particularly acute for women, who are led to believe that without beauty their value is diminished.
The film also touches on another theme - the denial of the real self. By sacrificing herself, Elizabeth gradually loses her individuality, changing with a younger and more perfect version of herself, but realises that behind this "perfection" there is an emptiness. This transformation is shown through visually metaphorical images: long corridors, slow motion scenes, silence and blank stares of the heroines.