You have 3 days to complete a series of quests to turn yourself into normal Link.
A skull kid wearing a mysterious mask, later revealed to be Majora's mask, steals the Ocarina of Time and turns you into a Deku Scrub! They knock you off your horse and you pass out. 2 fairies, similar to Navi, come and start bothering you. The story begins as you ride on epona through an unnamed forest as a kid. Out of any n64 game ever released, this probably has the most content, thanks to the expansion pack.