First, head to the town. Talk to the people and be sure to somehow buy a Long Sword - it helps a lot. Go to the tavern, chat with the people, get laughed at, fight the guards, and you're done inside there. Then head west of the tavern and enter the house. Offer to find the little girl named Mary. Don't try to go into the abandoned mine yet - it's suicide. Explore the country side picking up heart containers and other treasure. Once you feel secure follow the path south of Terris and once it branches west, go down a screen. You'll then reach the abandoned mine. Talk to the trapped guy and he'll tell you that you're going to need some kind of magic to free him. Head back to the tavern in town and the bartender will tell you about the old hermit that lives south of the tavern. Go to his house and knock on it by pressing space bar near the door. Go in and he'll teach you the Acid Rain magic if your magic ability is at least 5. If it isn't 5, go back to the mine and fight around to get it up to 5. Head back to the mine with the magic and free the man. The Cast will appear and a short automatic sequence will follow. The guy you freed will then die as he tries to fight the Cast and he'll give you the scroll.
Now head to Goodheart Castle and go just northwest of it. You should see two poles and if you try to walk through them you'll take damage and get knocked back. Use the scroll you received and the barrier will disappear, allowing you entry. Keep on going north, once inside, until you can go north no further. Then head east a screen and go north. Follow the path and you'll reach four girls worshipping a dead dragon carcass. Keep on listening and then you're going to have to fight the girls who are really Bonkas. After killing them the guy will turn into a Slayer - a very deadly creature at this point in the game. Stay at a safe distance and use Fireball magic to slowly wear him down. Now that Mary is saved you'll become a hero in Terris.