The title of this episode refers to the Ninth Circle of Hell in Dante Alighieri's Inferno, the first part of his 14th century three-part poem Divine Comedy. In the poem, Hell consists of nine concentric circles of suffering within the Earth, each circle representing a gradual increase in wickedness. Sinners end up in a circle that befits their sin. The ultimate sin, treachery, leads a soul to the Ninth Circle.