Zork III is somewhat less of a straightforward treasure hunt than previous installments. Instead, the player—in the role of the same "adventurer" played in Zork I and Zork II — must demonstrate worthiness to assume the role of the Dungeon Master. The player must get past the Guardians of Zork, with the complete garb of the Dungeon Master, and then endure a final test. The player must be wearing the amulet (found at the bottom of the lake), the cloak and hood (received when the player chooses to look under the hood of the vanquished opponent in the Land of Shadow rather than deal the final blow), the staff (received from the man at the Cliff for helping him retrieve the chest of valuables; attacking the man in any way or trying to take the valuables from him inevitably breaks the staff), the strange key (found in the Key Room when the player uses the grue repellent (from Zork II) to get past the dark places), the royal ring (retrieved from the Royal Museum by using the gold machine to travel back to the year 776 GUE), and the black book (found in the Royal Puzzle).
Unlike Zork I and Zork II there is a time-sensitive event, an earthquake which is randomly triggered about 130 turns into gameplay. The player must retrieve the key before the earthquake (when the Aqueduct is broken, leaving no escape route from the Key Room) and can't complete the Royal Puzzle or retrieve the ring until after the earthquake (when the unopenable Great Door breaks). Also unlike the previous two Zork games, the lantern is of little relevance. It is needed only to walk through the dark areas of the Junction, Creepy Crawl and Foggy Room at the beginning of the game (often done after completing the Land of Shadow, Cliff, and Flathead Ocean—saying "Hello, Sailor" to the viking yields an invisibility vial—tasks). Another light source, the torch from the Scenic Vista, is used to retrieve the repellent from Zork II and deposit it in the Damp Passage via the teleportation table to provide a light source for the return journey after retrieving the key.
Once the player has all the items, they must give the waybread to the elderly man in the Engravings Room to find the doorway leading to the final hallway. Here the "elvish sword of great antiquity" is used for the second and only time to block the beam in the Beam Room. Next the adventurer must get through the Guardians of Zork. This can either be accomplished by using the complicated Mirror Box or by simply drinking the invisibility potion in the vial from the Flathead Ocean. When the player knocks on the Dungeon Master's door he will only open it if the player is fully equipped (see above). He then tells the adventurer that he will obey their commands. The corridors lead to a parapet which overlooks the fiery cells. Reading the book here reveals a map of "The Dungeon and Treasury of Zork" which has 8 cells, one of which with a bronze door that leads to the Treasury of Zork. The eight positions of the dial in the parapet correspond to the 8 cells. The adventurer must use trial and error at this point to summon the cell with the bronze door and have the dungeon master return it to its original position by replacing it with any other cell. The key will now unlock the door revealing the Treasury of Zork, which contains the wealth of the Great Underground Empire as well as a controlling share in FrobozzCo International.
Get the lamp and turn it on. Go south. Have a look at your sword, stuck in the rock, but you can't take it directly. Continue south until you reach the Lake Shore. Turn the lamp off and drop it. Jump into the lake. Dive down. The "small, shiny object" or "something shiny" in the sand is an amulet, so take it. If you don't get it, keep trying. If you run out of air you just return to the surface, but you'll probably need to exit the lake and re-enter it lest you freeze to death from staying in the water too long. When you have the amulet, go to the western shore, and then go south. Get the torch and wait until the indicator over the viewing table reads "II". Touch the table. You'll find yourself in Room 8 from Zork II. Get the grue repellent and wait one turn; you will return to the Scenic Vista. Wait one turn (the indicator reads "III"), then touch the table again. Damp Passage is a location one south and one east from the Endless Stair at the start of the game. Drop the torch here and again wait to be returned to the Scenic Vista. Return north to the lake shore and jump in once more. Dive, retrieve the grue repellent you dropped, and swim across to the southern shore. Apply the grue repellent to yourself and go south, south, and east. Get the key. Raise the manhole cover and you will find an exit down to the Aqueduct. Go north three times to the Damp Passage and get the torch. You can return to the Lake Shore for the lamp if you like (nostalgic?), but the torch will last the rest of the game.
Go west three times to the cliff-top and get the waybread. Go down the rope. Get the chest. Wait on the ledge for a man to appear at the top of the cliff. Tie the chest to the rope, and wait again until he reappears. Grab the rope. The man will give you a wooden staff from the chest. Head back down the cliff twice, and go south to the Flathead Ocean. Wait until you see a ship approaching. Say hello sailor (you were waiting for the game where that would do something, wouldn't you?) and the Viking will throw you a gift. Pick up the vial and examine it - it's an invisibility potion. Go east. Wander about in the Land of Shadow until you encounter the hooded figure. Your sword will appear in your hand. Attack the figure repeatedly, but don't kill it. When the figure is "badly hurt and defenceless", take its hood. Take its cloak as well. Find your way east out of the Land of Shadow to the Creepy Crawl.
Go east twice and south twice. If the earthquake hasn't happened yet, wait here until it does. Go east through the cleft in the rock to the Museum Entrance. Go north. As will become apparent soon, the grey machine is a pressurizer like the one used in Zork I to make the diamond, and the black machine is a room spinner like the one in Zork II. Push the gold machine (a "temporizer", or time machine) south. Open the stone door and push the machine east into the Jewel Room. Read the plaque there. Dedicated in 777, huh? Turn the dial on the gold machine to 776, then sit on the seat and push the button. You are now in the Jewel Room in the year 776. Wait two turns until the guards leave. Take the ring. Open the door, go west and open the wooden door, then go north. Look under the seat of the gold machine. Put the ring under the seat. Sit in the machine again, turn the dial to 948 (the present, when you found it in the first place), and push the button. Look under the seat and you will retrieve the ring. Stand, open the door, and go south. Open the stone door and go east to the Jewel Room. Get all your belongings. Read the plaque again.
Go west and south to the Royal Puzzle entrance and read the note. ("Intrepid adventurers do not fear mere warning notes!") Go down into the Royal Puzzle. The sandstone "walls" are actually the sides of cubical blocks. In here, you need to get the lore book (which is hiding under one of the blocks), and then manoeuvre a ladder (which is attached to another one of the blocks) to allow you to climb back out at the place you came in. There are two blocks with ladders, but only one can be positioned correctly. The following sequence of moves will work:
PUSH EAST WALL. S. S. SE. PUSH SOUTH WALL. N. NE. PUSH SOUTH WALL. TAKE BOOK. PUSH SOUTH WALL. E. NE. PUSH WEST WALL. SW. NW. NE. PUSH SOUTH WALL. SW. PUSH EAST WALL. NE. PUSH SOUTH WALL. NW. N. N. N. PUSH EAST WALL. SW. S. SE. NE. N. PUSH WEST WALL. NW. PUSH SOUTH WALL. AGAIN. W. NW. NW. PUSH SOUTH WALL. SE. SE. SE. NE. PUSH WEST WALL. AGAIN. SW. PUSH NORTH WALL. AGAIN. AGAIN. NW. UP.
Leave the Museum and go to the Engravings Room: north, west, north, north, west, west, north, east, northeast. If no one is there, leave the room and return until an old and wizened man appears. Shake the man to wake him, and then give him the waybread. Open the secret door and go north twice to the Beam Room. Drop the can of grue repellent, to block the beam of light. Go south and press the button.
Return north thrice and you will now be inside the Mirror Box. Raise the short pole so you can rotate the box, and push the yellow wall twice (the compass rose now points north). The compass rose points the same direction as the mahogany wall, which allows you to move the box when you push it. Lower the short pole again and push the mahogany wall three times. Raise the short pole. Push the white or black walls to rotate the box so the mahogany wall faces south. Push the pine wall and go north to the Dungeon Entrance.
Knock on the door and the Dungeon Master will answer. Go north, east or west, and north twice. Look at the sundial. Turn the dial to 4, and then press the button. Go south, open the door and enter the prison cell. Tell the Dungeon Master to go to the Parapet, then to turn the dial to another number (pick any), then press the button. Unlock the bronze door with the key, and then open the door. Go south into the Treasury of Zork. Congratulations, Dungeon Master!