while on an expedition, katara and sokka discovered aang inside an iceberg with appa. after katara freed him, sokka initially believed that the aang might be a fire navy spy, and he eventually grew xenophobic enough to banish him from the village. upon discovering that aang was the avatar, destined to master all four elements and bring peace to the world, sokka relented, realizing that they had a common enemy in the fire nation. he then set off with katara and aang on their journey to find a waterbending master at the north pole, determined to hone his warrior skills by way of fighting firebenders along the way. fire nation general zhao killed tui (the moon spirit) causing the moon to turn red and then vanish as during a lunar eclipse, thus preventing the waterbenders from using waterbending. princess yue (one of sokka's love interests, the other being kyoshi warrior suki) then becomes the new moon by giving her life energy to the moon spirit, thereby restoring the moon in the sky and the abilities of the waterbenders.