World 1-2 is the second level of World 1, and subsequently, of New Super Mario Bros.. Completing this level through the normal exit will unlock World 1-3, whilst exiting the level from the secret exit will give the player access to World 1-Tower.
Pursuing the chase after Bowser Jr., Mario finds himself in a strange cave. Adventuring on, Mario finds that many Goombas and Koopa Troopas inhabit this cave. He then sees some ? Blocks. Hitting it produces Coins. Mario collects them, and moves on to a strange sight - a Star Coin. He jumps onto the ledge, and collects it. He could also Ground Pound on some brick blocks to collect a bunch of coins. He then moves on, encountering more Goombas. Going forward, Mario finds a block of rock that he can't get through - yet he prevails by smashing the brick blocks above it, and getting through. An abyss is then met, Mario easily jumps over it, but then meets a Star Coin surrounded by some brick blocks. Trying to find out how to get in, Mario jumps to the top and Ground Pounds it - the blocks are destroyed, and Mario gets the Star Coin. A Piranha Plant is then met. Mario jumps over it, and goes down the pipe it was guarding. Mario, finding a passageway, collects the Coins, then hits the Koopa Troopa to break some brick blocks, creating a passageway. Mario crawls under the passageway - he then comes across, and finds some Goombas. Running, he then sees a ? block. A Mega Mushroom pops out. Mario collects it, and destroys all the Goombas and brick blocks. He then goes up a pipe nearby. Mario is then in the main cave again, and jumps carefully over some pipes sticking out of an abyss. He then makes it onto the ground, and runs forward. He then meets a tilting log. He runs over it, and continues through the cave. A pipe then greets him, as he appears out of the pipe, and into the sunshine again, and also near the Flagpole. He then finishes the level. There is an Invincibility Starabove the first Brick Block after the first Piranha Plant.