The Moros was a Monan Host ship on which Romana II was sent on a mission to fix its engines. While the Moros was at its closest point to Gallifrey, Gaal planted a computer virus in the computer that caused it to kill its crew and begin a core meltdown. While President, Romana II was sent on a mission to save the ship's engines; though she wanted to go alone, Leela secretly followed her.
Originally, Romana tricked Leela to leave the Moros and go to Legion while Romana regenerated to create the energy needed to prevent the core from collapsing. However, when a war began after the Omega incident, Romana III sent Irving Braxiatel back to stop her from regenerating at that time.
In the resultant timeline, Braxiatel brought Romana II to pick up Leela and return to Gallifrey, where they shut down the Adherents of Ohm. However, the Moros engine core collapsed and became a singularity that grew into a black hole that threatened to swallow Gallifrey. The Monan Host, who believed that the Time Lords had planted the computer virus, left their embassy in the Capitol in preparation for war until Gaal was exposed as having planted the virus as part of a plot with Kalbez and Plutus.
Finally, Romana II from the changed timeline and Leela from the original timeline were persuaded by the Watchmaker to return to the Moros right after Braxiatel rescued Romana; there, they gave their lives to stop the singularity from growing into a black hole. As a result, the Moros was not destroyed. (AUDIO: Enemy Lines)