Princess Leia Organa has somehow acquired a data cube containing Imperial battle plans, but before she could transmit the plans to the Rebel Alliance, her transport was shot down on Nibiru.
Luke Skywalker was sent to Nibiru to find and rescue Leia. Imperial agents were also on the hunt for Leia, but Luke managed to evade them and find Leia in an Ewok tree village, where she was being tended to by the locals.
The data cube containing the Imperial battle plans was still aboard Leia's transport, however, which had crashed into a lake and was now partially underwater. With the help of a breath mask, Luke boarded the wreckage and recovered the battle plans, which Leia promptly transmitted to Mon Mothma.