Haakon Haakonsen, a young Norwegian boy in the 1850s, becomes the family's sole breadwinner when his father is injured. He signs on as a cabin boy aboard a ship, where his bond with Jens — who acts as a compassionate “sea daddy” and elder brother — helps him navigate the harsh, unfamiliar world of seafaring. Haakon’s early days at sea are rough, but with Jens’s steady guidance he gradually earns the respect of the crew and finds his footing among sailors who test his resolve.
When the ship makes a shore stop in London, a British naval officer appears on deck under the pretense of bringing protection from pirates. The man, known on the crew as the enigmatic Lt. Howell, is secretly the notorious pirate John Merrick, who has murdered the real Howell and stolen his identity. Haakon soon uncovers that Howell has secretly smuggled guns onto the vessel and tries to persuade the boy to stay quiet about them. The tension thickens as the crew remains unaware of the deception, and Haakon begins to suspect that danger stalks them from within.
The voyage takes a grim turn when the admired captain is poisoned, and Howell seizes command amid growing unease. In Sydney, a fresh batch of sailors arrives, and shore leave is abruptly canceled for reasons none of the crew can fully fathom. After they leave Australia, Haakon discovers a stowaway on board: a girl named Mary, who becomes more than a passing acquaintance. The presence of Mary unsettles the ship’s already fragile balance, and when the new captain discovers Mary’s presence, he demands that anyone involved with her reveal themselves. Haakon steps forward, taking responsibility, and the captain sentences him to be lashed with the cat o’nine tails. Jens protests, arguing that punishment for Haakon could mean death, but Howell orders Jens to administer the lash instead of himself. Their predicament is interrupted by a devastating storm that cripples the ship and tears the mainmast apart, leaving the crew in disarray.
In the chaos, Haakon manages to rescue Mary from the brig, but the ship sinks, and he wakes alone on a deserted tropical island. He searches the island and uncovers a cache of treasure and wanted posters bearing the image of Merrick, the pirate who bears an uncanny likeness to Howell. Haakon learns that the newly arrived shipmates are, in fact, pirates who hid treasure on the island during a prior chase by naval forces, and they plan to return for their loot.
With stubborn determination, Haakon trains himself in swordplay and marksmanship, and even fends off a gorilla by using a horn to establish dominance. The ape gradually becomes a companion, almost like a pet, as Haakon builds a defense with a series of clever booby traps designed to deter any pirate return. Although he grows more confident on the island, he cannot help but miss Jens and his family in Norway.
Hope arrives in the form of a distant smoke signal. Haakon builds a raft and makes a perilous voyage to the nearby shore, where he finds a village of natives performing a nighttime dance. Misunderstandings erupt when Haakon, seeing Mary among the natives in native dress, fires a warning shot in fear. The moment nearly turns tragic, but Jens arrives to explain that the locals are peaceful and that they, in fact, saved Haakon’s companions earlier. With this understanding, Haakon, Mary, and Jens depart together the next day.
Soon after, the pirates descend again, bringing Berg and Steine aboard, two of Jens’s former shipmates who survived the sinking. Haakon’s booby traps prove insufficient against the resurgent threat, so Mary takes a bold risk: she sneaks onto the pirate ship, frees the remaining crew, and helps them overpower the guards. Back on land, Haakon and Jens work to divert the pirates long enough to free Berg and Steine, and they manage to retake the ship and escape, leaving the pirates stranded on the island.
In a final return to their homeland, Haakon, Jens, Mary, and the liberated crew sail back to Norway. Each man pockets a share of the treasure, and Haakon vows to use his portion to lift his impoverished family out of debt. Back home, Haakon reunites with his family and introduces Mary, who is welcomed to stay until she can reestablish contact with her own relatives.
The tale closes with a hopeful sense of restoration: a boy’s resilience, forged through hardship at sea, helping rebuild a family and a future, while friendship and loyalty prove stronger than the treachery of pirates and the tempests of the ocean.
