Ishmael is a young man who, in response to feelings of restlessness and depression, decides to go to sea. He makes his way from his home in Manhattan to Nantucket, Massachusetts, making a friend in Queequeg along the way. The two ship out together on Pequod. Ishmael tells the story of Ahab's mad quest to kill Moby Dick from his own perspective—both as a young and relatively experienced crewman on the Pequod and, looking back, as the sole survivor of the Pequod's encounter with Moby Dick.
Captain Ahab is the protagonist and tragic hero—complete with a fatal flaw—of the novel. Several days after the Pequod sets sail, Captain Ahab finally emerges from his cabin. His appearance—a leg made of whalebone, a long, ugly scar—as well as his intense demeanor—add to the mystery surrounding him. Ahab's arrogant quest for vengeance against the White Whale drives the events of the plot, as the fates of all aboard the Pequod are tied to the outcome of Ahab's mad mission.
Moby Dick is the "White Whale" that took Ahab's leg and upon whom Ahab wishes to take vengeance. Ahab believes Moby Dick to be the physical form of evil and malice, yet symbolically the White Whale represents the unknowable nature of God, the absolute power of the natural world, and the inevitability of fate.
Queequeg, despite his fearsome tattoos and his tendency to perform everyday activities with his harpoon, becomes a friend to Ishmael. He performs his duties on the ship without complaint, even when they are dangerous, and he is enthusiastic about harpooning. As a pagan and a cannibal, his friendship with Ishmael helps develop ideas about spirituality and culture.
As a Quaker and the only man aboard the Pequod with serious moral reservations about Ahab's quest, Starbuck's growing discomfort with Ahab's obsession with revenge causes him to nearly kill his captain. He holds back based on moral reservations and thoughts of his wife and child. Ultimately, his hesitation to commit murder leads to tragedy.
Although Stubb is humorous, his comic approach is grounded in something more serious: he is a fatalist. He believes there is little humans can do to change the course of events, so a person might as well laugh about it all. This also explains his relaxed approach, whether in pursuit of a whale or smoking his pipe.
Flask is not a very complicated man. He lives to kills whales, and he pursues that purpose with a zeal that can be unsettling. He cannot see anything majestic about the giant creatures. Indeed, Flask's whole world is black and white; he is not able to see nuances. This makes him an easy target for other crew members who sometimes trick him with their words just for fun.