The Shadow-Line, a compact novel of compressed action, puts Conrad's unnamed young captain (who is himself, and thus the novel's subtitle: A Confession) through all the paces seen variously in other of Conrad's work. To breach the "shadow-line" between youth and whatever comes after youth, the protagonist must face more than one "double," the confusions of officialdom, a crew that doesn't know him, disease, and a ship becalmed in hell—and all this on his first outing as a captain.
The first act suggests we'll not be impressed by our "hero." His judgment of others is clouded by an unsourced petulance; he is quick to anger. And his career seems to have stopped before it has much begun, as he has sworn to leave the sea-going life behind for something more ordinary that he cannot even imagine. This serves as a long prelude to his adventure. Structurally, it's odd, a great deal of time spent returning our young man, much to his surprise, to the waters he's just left, having been awarded his first command without having gone through a long career of waiting. The opening confusions may be demonstrating that the protagonist's true place is the ocean, aboard ship; on land, he struggles to navigate the signs and speeches of people. We also see that he is not immediately empathetic with others, that he can't pierce their behaviors to grasp their personal circumstances.
Once aboard ship, he has, after the awe about his situation has lessened, more connection with the people who surround him: they have a mission, a set of tasks, and they are all men of the sea. The former captain haunts this posting. According to the first mate, the previous captain was a horror, playing violin rather than attending to the ship, growing more mad when the ship ran into trouble, and finally wishing doom on the vessel and all its men. How could such a person have ever led a crew? (I'm reminded of the frequency with which Star Trek's Kirk encountered starship captains who'd lost their minds or violated the Prime Directive; after a few episodes, one became convinced that the Enterprise was one of the few sanely managed ships in the Federation.)
On this haunted, cursed ship (for the first mate believes the late captain clings to it like a demonic anchor), the crew takes sick only a few days out, the vessel is trapped on an oppressively hot, utterly windless path of ocean for weeks, the medicine (likely sold ashore by the previous captain) is discovered to have been replaced by worthless powder, and the awaited winds, when they return, threaten to ruin the seriously undermanned ship. Sea and sky alike become an existential blank in which young Conrad looks to find meaning but is repeatedly confronted by an absolute and impersonal void. Our captain endures—and succeeds—through his noble yet messy bond with the men and his devotion to duty, bringing all hands safely home.
Though the novel takes place (given the autobiographical elements) in 1888, it was composed in 1915, at the start of the Great War, and dedicated to Borys, Conrad's eldest son, who had received a commission as a second lieutenant. The characters in the novel are not at war, but evidently Conrad intended to convey that, regardless of the circumstances, to be at sea is to be in some grand conflict. One's primary challenge is to live, and one can only do that with the support—and it is implied, in support—of others. Returned to shore, he is given some perspective by the captain who clued him in, initially, to the possibility of a command. This thoughtful mentor concludes that "a man should stand up to his bad luck, to his mistakes, to his conscience, and all that sort of thing. Why—what else would you have to fight against?"