The TARDIS lands on the planet Mendeb, where Ace meets a merchant adventurer named Kedin. She falls for him, seduces him -- and then discovers him to be a slave trader. When, horrified, she tries to flee, he feeds her the will-sapping spore drug that turns independent citizens into obedient slaves, and sells her. The Doctor is on a mission to liberate the Mendeb slaves and lead them in revolt against the emperor Vathran -- but can they survive without the support of their generous employers? Meanwhile, Ace finds herself sold into the henads of Kedin's arch-enemy Vathran, where she learns the reason for Kedin's slave trading -- Vathran has imprisoned Kedin's family and is threatening to have them put to death.