Upon arriving at the Isle of Ember, disembark the S.S. Linebeck. The island is volcanic and filled with various enemies. Your primary goal here is to find the entrance to the Temple of Fire. Follow the path, defeating any Octoroks or other creatures you encounter. You'll notice some boulders blocking paths; you won't be able to move these yet, so ignore them for now. Eventually, you'll come across a cave entrance. Enter the cave, which leads to the Temple of Fire. Inside the cave, you might find some pots with Rupees or hearts. Be mindful of the lava pits; falling in will cause damage. The path through the cave is relatively linear, leading you directly to the temple entrance. Once you reach the large door, interact with it to enter the Temple of Fire.
