Taken from Wikipedia.org:

The player begins in the Barrow from Zork I armed only with the trusty brass lantern and the elvish sword of great antiquity from before. The objective of the game is not initially clear. The Wizard of Frobozz is soon introduced. The wizard was once a respected enchanter, but when his powers began to fade he was exiled by Lord Dimwit Flathead. Now bordering on senility, the wizard is still a force to be reckoned with. The player's goal in the wizard's realm is to avoid his capricious tricks and to learn to control his magic.

Zork II has two notoriously[citation needed] difficult puzzles: the Bank puzzle, in which the player must figure out how to walk through several bank walls in order to figure out how to get out of bank with the loot from the vault without tripping the security alarm; and the Oddly-Angled Room, a diamond-shaped room filed with panes of glass representing baseball bases. The player must figure out how to traverse the diamond in order to open a door that will lead to the game's finale.

Like its predecessor, Zork II is essentially a treasure hunt. Unlike the previous game, the ten treasures are tied together by a crude plot. Finding the treasures does not end the game, nor are all the treasures needed to finish the game. Instead, the adventurer must figure out a way to use the treasures in order to reach the game's finale.

Zork#Zork_II:_The_Wizard_of_Frobozz