After defeating Ganon in Ocarina of Time, Link rides Epona through a forest when he gets kicked off and his Ocarina gets stolen. He follows the skull kid, who stole it, and ends up a place called Termina. The skull kid turns him into a Deku shrub, so the first thing Link has to do is turn back to normal, but he only has three days to do this, because after three days, the moon will crash into Termina, killing everyone. After turning back to normal and getting back you Ocarina, you can travel back through time to when you first got to Termina, starting the three day cycle again. All the characters in the game have a certain path they walk these three days, so there's a lot to see and a lot of schedules to keep up with. If you need to talk to a character you have to know where he is at a certain time. The game only has four dungeons, but having to save the world in three days makes this one of the harder Zelda games.