The search-and-rescue operation on Nibiru was a task completed by Luke Skywalker throughout the Galactic Civil War.
Somehow, Princess Leia Organa came into possession of a data cube that held Imperial battle strategies. Before she could send these strategies to the Rebel Alliance, her transport was shot down, crashing on Nibiru.
Luke Skywalker's assignment was to locate and save Leia on Nibiru. While Imperial agents were also searching for Leia, Luke successfully avoided them and located her in an Ewok village built in the trees, where the local Ewoks were providing her care.
However, the data cube, which contained the Imperial battle plans, remained on Leia's crashed transport. The transport had gone down in a lake and was partially submerged. Luke, using a breath mask, entered the wreckage and retrieved the battle plans, which Leia then transmitted to Mon Mothma.