The reason because of which Captain Ahab is after the white whale is because that white whale once bit off Ahab's leg in his previous expedition on the sea because of which he now has to wear a whalebone prosthetic leg.
In Herman Melville's Moby-D***, Captain Ahab is a fictional figure and one of the primary protagonists (1851). He is the obsessive captain of the Pequod, a whaling ship. The Pequod's whaling expedition turns into a quest for ve-nge-ance against the whale as Ahab coerces the crew into aiding his obsessive cause. Ahab loses all sense of caution when Moby D*** is finally seen due to his hat-red, and the whale pulls him to his end under the water.
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