Letrus was the moon of Ballustra in E-Space.

A number of Ballustrans were murdered by Farrian who were using Letrus as a base. The Farrian used the moon to scan Ballustra for minerals.

Romana II and Adric were taken to Letrus after being arrested. There, they were questioned by Marni Tellis. The Farrian showered both Letrus and Ballustra with teleport disks. These sent individual Farrian onto the surface. The Fourth Doctor rescued Marni from this invasion. (AUDIO: The Invasion of E-Space)