We open some sixty years after the events of "The Hobbit" -- Bilbo Baggins is older, not much wiser, substantially wealthier, and quite eccentric (one not-so-affectionate nickname is "Mad Baggins"). He has also adopted his bright young cousin Frodo, who was orphaned at a young age and had led a rather fractured life since then. On his 111th birthday, Bilbo suddenly vanishes, leaving behind all his possessions to Frodo -- including the golden ring that allows its wearer to become invisible.
Seventeen years later, Gandalf the wizard shows up again on Frodo's doorstep, and informs the young hobbit that his ring is in fact the One Ring of the Dark Lord Sauron. It inevitably corrupts those who have it, and most of Sauron's power is invested in it. Trying to deflect danger from the Shire, Frodo leaves with his best friend Sam and his loyal cousins Merry and Pippin. But Frodo has only the slightest idea of the hideous and dangerous journey ahead of him, that will take him across Middle-Earth to the evil land of Mordor.