Ishmael believes he has signed onto a routine commission aboard a normal whaling vessel, but he soon learns that Captain Ahab is not guiding the Pequod in the simple pursuit of commerce but is seeking one specific whale, Moby-Dick, a great white whale infamous for his giant size and his ability to destroy the whalers that seek him. Captain Ahab's wooden leg is the result of his first encounter with the whale, when he lost both leg and ship. After the ship sails it becomes clear that Captain Ahab is bent on revenge and he intends to get Moby-Dick.
Ahab demonstrates erratic behavior from the very beginning and his eccentricities magnify as the voyage progresses. As the novel draws to a conclusion, the Pequod encounters the whaling ship Rachel. The Rachel's captain asks Ahab to help him in a search and rescue effort for his whaling-crew that went missing the day before -- and the captain's son is among the missing. But when Ahab learns that the crew disappeared while tangling with Moby-Dick he refuses the call to aid in the rescue so that he may hunt Moby-Dick instead.