Sabin floats down the Lete River/Lethe River after getting separated from the others following the battle with Ultros.
Meet a New Character
Sabin will start his scenario in the overworld, near a house. Go to that house. A merchant will ride up on a Chocobo. Don't talk to him yet. Instead, talk to the guy who looks like a ninja. (Default name: Shadow). He decides to help you reach Narshe. I recommend saying Yes when asked if you want him to join you.
After Shadow joins you, be sure to save the game often. He wasn't kidding when he said that he might leave at any time. If he does leave, you can start the game from the last save point to get him back.
Talk to the merchant (if he's not there, go into the house and then outside again). Be sure to stock up on Tonics/Potions and Fenix Downs/Phoenix Downs before you go. Also, since Shurikens are so cheap, buy as many Shurikens as you can carry. Shadow can use them with his Throw command. Sprint Shoes will make you walk more quickly. I highly recommend buying some.
43. CrassHoppr/Crass Hopper/Nettlehopper
Go to the Base
Leave the old man's house and go all the way east. Then go south through the forest. Walk onto the patch of desert to arrive at the Imperial Base/Imperial Observation Post. The enemies that you encounter at the Imperial Base/Imperial Observation Post cannot be encountered later, so be sure to fight them if you don't want to miss any.
Go south and watch the cutscene. You will meet a new character (default name: Cyan).
Cyan has the SwdTech/Bushido ability in battle. He knows three SwdTechs/Bushidos now, and will learn more as he levels up. To use this ability, allow his meter to reach the desired number. Technique 1 is Dispatch/Fang, which attacks a single enemy. Technique 2 is Retort/Sky, which causes Cyan to counterattack the next time he is attacked. Technique 3 is Slash, which halves a single enemy's HP and inflicts Seizure/Sap on that enemy.
When Cyan is standing in front of the castle, go south and talk to the soldier wearing dark armor.
Cyan should use SwdTech/Bushido 1 on every turn.
After the battle, you will be back in the Imperial Base/Imperial Observation Post. You can leave and save the game if you wish. Then go back in.
Back inside the Base, go into the tent to the right. In order to not miss one of the monsters in the game, kick the chest. You will fight Doberman. Otherwise, if you want to skip the battle, hit the chest. Then open the chest to get a Star Pendant.
Leave the tent and go south to see another cutscene. Afterward, go south again to see another scene. Eventually, you will fight Kefka, but all you have to do is hit him once and he will run away.
Don't talk to Kefka again yet. (This is important if you want to collect everything in the game.) Instead, go into the tent west of here. Open the chests. The chest on the left contains a monster that is called Telstar/Satellite. If you choose to fight it, you can win a Green Beret, which is the best helm available at this point in the game. Fighting Telstar/Satellite also makes it possible to get even more Green Berets later.
To defeat Telstar/Satellite, have Sabin use normal attacks (using a Blitz would cause Telstar/Satellite to inflict Berserk status on Sabin), and have Shadow throw Shurikens at it. If soldiers appear, ignore them and continue attacking Telstar/Satellite.
After this, talk to Kefka until there is a cutscene. (If you have to battle him, just use Fight to hit him once and end the battle.)
After the cutscene, when you are in control of Cyan, go north into the castle, go down the stairs, then go south and approach the Throne Room. After the cutscene, leave the Throne Room and enter upper-right door, then watch the cutscene.
Go up and talk to Cyan to help him fight. Help him through several battles until you are riding Magitek armors. Use the armors to heal yourself during battle.
Go east, then north, fighting battles as you go. After the final battle, you will automatically exit the base via the north exit where you first entered.
In the overworld, go across the bridge to the west, then go south and cross another bridge, then go south into the forest.