Ys (イース, Īsu?) are two different anime series, both of which are based on the video game series Ys. The first series (often referred to as "Book I") spans seven episodes and covers the general plotline of the first game of the series, Ys I: Ancient Ys Vanished. The plot centers on Adol Christin as he journeys to the island of Esteria in search of adventure. Upon arrival, he learns that a dark priest known as Dark Fact has been tearing the island apart in search of six mystical tomes of power, the Books of Ys.

The second series (referred to as "Book II") picks up immediately after the end of the first and follows the second game, Ys II: Ancient Ys Vanished - The Final Chapter. With Dark Fact's apparent defeat, Adol sets out to solve the mystery of the mysterious books of Ys by going to the source: the floating island of Ys itself. Along with Feena and Reah, Adol makes the journey to the floating island and picks up another companion, Lilia. Also present is Dalles, attempting to resurrect the demon god Darm once again.

Source: Wikipedia