When Sailor Moon and her group arrived at Sagittarius Zero Star, they were confronted by unbearable heat, as the area surrounding Shadow Galactica was all desert. They came upon a river with a mysterious boatman who offered to ferry them across. This was a trap, though, and Usagi awakened at the bottom of the river, with no memory of who she was or why she came. Sailor Lethe revealed herself there, and she attacked the unsuspecting Sailor Moon.
Unlike most of the Sailor Senshi under the control of Sailor Galaxia, Sailor Lethe and Sailor Mnemosyne were true Senshi, and they felt they had no choice but to follow her. Before Galaxia's arrival, the planets of Lethe and Mnemosyne were always at war with one another, and the chaos resulting from the struggles was uncontrollable. Sailor Lethe and Sailor Mnemosyne joined Galaxia with the hope that she could bring about a better future for the universe, and end all war once and for all.
Of the two Desert River Senshi, Lethe was the more aggressive one. Not only did she attack Sailor Moon while she was unable to fight back, but she also killed Luna, Artemis, and Diana in order to make her point about how futile Sailor Moon's life was. Lethe's vengeance was thwarted by her sister, Sailor Mnemosyne, who inadvertently allowed Sailor Moon to remember who she is (a difficult feat, given that she had swallowed water from the Desert River of Oblivion) and resurface with Sailor Chibi Chibi. Mnemosyne also helped to revive the memories of Princess Kakyuu with water from the Desert River of Memory. Despite her betrayal, Lethe could not bring herself to attack her sister, as they both shared the same beliefs and convictions; instead, she reassured Mnemosyne that Sailor Galaxia would make everything all right for them.
Lethe held a strong belief that Sailor Moon and the Silver Crystal were the cause of all of their troubles, and said as much to Sailor Moon and Princess Kakyuu when they confronted each other. Lethe's belief in Galaxia was then shattered when Sailor Moon offered to Lethe and Mnemosyne the simplest solution to their problems: to kill her. Sailor Moon's offer of self-sacrifice changed both Lethe and Mnemosyne's minds about Sailor Moon's true nature, and they realized that Sailor Moon came to Shadow Galactica to end war, not to cause it.
Just as Lethe and Mnemosyne allowed Sailor Moon's party to pass through, Sailor Phi and Sailor Chi appeared and killed them, taking the Sailor Crystals of the Sailor Starlights as well as Lethe's and Mnemosyne's.