The main story is framed by fictional scenes of Herman Melville visiting the Nantucket home of Tom Nickerson, the last living survivor of the voyage of the "Essex".  Nickerson is beyond reluctant to recount what happened, clearly still traumatized by an experience he deems to be unspeakable, but Melville's persistence, the insistence of Nickerson's wife, a tidy sum of money, and enough liquor to help him get through it, are enough to ultimately get him to open up and tell his story for the first time to anyone. Nickerson, played as a young cabin boy by Tom Holland (60-year-old Brendan Gleeson isn't remotely passable as a man in his early forties), relates of the story of "Essex" First Mate Owen Chase, a seasoned seaman worthy of being captain who had to settle for being second in command for one more adventure due to being "of low birth" as the son of a farmer as compared to the relatively less experienced captain of the ship, George Pollard, who comes from a privileged family of naval adventurers. (How Nickerson knows of all of the events that led up to his actual participation when he never wanted to relive or discuss them with anyone is one of the problems with the needless narrative framing device).  Their mission is to venture out and bring back 2000 barrels of whale oil, but Holland is clearly over his head in command, and gets little help from Chase after chiding him for his social status persistently.  Calamities abound, but they stubbornly refuse to come back without their full shipment of oil, which sends them off to the whale-rich waters of the southern Pacific Ocean, where they encounter the rumored 100-foot whale thought too massive for any whaling vessel to take down.