Plot summary
Gabrielle turns leadership of the Amazons over to Ephiny, but the celebration ends when Velasca returns. She survived their last confrontation and has obtained ambrosia. Eating it heals her injuries and transforms her into a goddess. She attacks Gabrielle for taking the Amazon throne, forcing Xena and the Amazons to retreat.
Velasca is still learning to control her powers and briefly weakens after using them. Xena realizes they have only a short time before she becomes impossible to contain. Needing an immortal opponent, she proposes freeing Callisto from the prison where Hercules trapped her. Gabrielle is appalled at working with the woman who murdered her husband, but there is no time to find Hercules.
Xena offers Callisto a chance to obtain ambrosia in exchange for fighting Velasca. Callisto accepts, first demanding that Xena publicly confess her responsibility for destroying Cirra. Xena makes the confession, but it brings Callisto no peace. Their alliance remains dangerous: at their first encounter with Velasca, Callisto briefly tries to unite with her against Xena and Gabrielle. Velasca attacks Callisto as well, pushing her back onto Xena's side. Together they bury Velasca beneath fallen rocks and gain time to prepare another trap.
During the night Gabrielle tries to understand whether Callisto feels anything about her crimes. Callisto answers with cruelty rather than remorse. Xena leads them toward a lava-filled chasm, using Gabrielle as bait to draw Velasca across a rope bridge.
In the fight, Gabrielle gets hold of Velasca's remaining ambrosia and throws it to Callisto. Callisto eats it and becomes a goddess too. As the two immortals battle on the bridge, Xena cuts its supports. Both fall into the lava. Xena catches Gabrielle before she follows them and gets her to safety.
With the bridge destroyed, the friends must take the long route out. Gabrielle still wants to believe that Callisto might feel remorse, because forgiveness otherwise seems impossible. Xena does not offer her that reassurance, but accepts that Gabrielle needs to work through the question herself.
Credits
Adapted from Legendary Journeys Wiki contributors, A Necessary Evil. Complete episode narrative, rewritten and condensed for clarity; causal developments and the episode's ending retained. Illustrations, dialogue quotations and non-story metadata omitted.