what is the plot in the story of Grendal..?

Answer 1

Do you mean Grendel?

Grendel is a beautiful and heartbreaking modern retelling of the Beowulf epic from the point of view of the monster, Grendel, the villain of the 8th-century Anglo-Saxon epic. This book benefits from both of Gardner's careers: in addition to his work as a novelist, Gardner was a noted professor of medieval literature and a scholar of ancient languages. (amazon)

Answer 2

There are three established incarnations of Grendel in literature. We've got bits about them, some papers, and links to other sites with more thorough information:

Beowulf -- The original Grendel was a monster in the Anglo-Saxon epic Beowulf. Descendant of Cain, mere-dweller, and with the meanest mother around, he was the hero Beowulf's first foe in the work.
Gardner's Grendel -- In 1972 (?) John Gardner published the novel Grendel, a retelling of Beowulf from Grendel's point of view. He portrayed the "ridiculous hairy beast torn apart by poetry" sympathetically, and walked him through the Great Concepts of western civilization, considering them from the outside with an existentialist edge.
Wagner's Grendel -- In the back of issues of his comic Mage, Matt Wagner began a storyline about criminal mastermind, assassin, anti-hero Hunter Rose, a.k.a Grendel. After a single short book and several separate short story arcs about Hunter Rose, the series spun out into a 50 (?) issue epic spanning centuries and the world.