Return of the Obra Dinn

The Obra Dinn, an East Indiaman under the command of Captain Robert Witterel, departs with 52 crewmen and 8 passengers from Falmouth, Cornwall, to the Orient in 1802. Among the ship's passengers are Witterel's wife Abigail; musician Nunzio Pasqua; wealthy Englishwoman Jane Bird; and two Formosan nobles and their guards transporting an exquisite chest. The ship fails to keep her rendezvous at the Cape of Good Hope and is declared lost. Five years later, the vessel reappears off the coast of England with every soul dead or missing. The East India Company sends a newly appointed chief inspector to determine what happened.

The inspector receives a copy of the Obra Dinn's logbook, drawings of the passengers and crew, and the Memento Mortem—a magical pocket watch—from the ship's surgeon Henry Evans, who lives in Morocco. The Mortem, when used on a corpse or its traces, enables the user to observe the exact moment of the corpse's death, frozen in time.

Early in the voyage, a seaman and a stowaway are crushed to death by unsecured cargo, and two seamen die of pneumonia. As the ship passes the Canary Islands, Pasqua discovers the second mate, Edward Nichols, skulking after the Formosans' chest in the cargo hold. Nichols stabs Pasqua dead and frames one of the Formosan guards. Witterel, bound to Company regulations, has the guard executed by firing squad. Nichols and a small party of defectors steal the chest, abduct the Formosan nobles, and make for the Canaries in the ship's two jollyboats.

Three mermaids ambush and kill most of the group. One of the nobles uses a magical shell pulled from the chest to stun the mermaids at the cost of his life. Nichols, now the sole survivor, captures the mermaids and returns to the Obra Dinn with the chest and the dead nobles, but is fatally shot by the surviving Formosan guard as he approaches. As they are brought aboard, the mermaids, who hold shells of their own, lash out and kill several crewmen before they are locked away in the lazarette. Witterel's steward Fillip Dahl kills a seaman and proclaims that the mermaids are cursed, so he is locked in the lazarette alongside the mermaids and the chest.

Captain Witterel orders the ship to return to England. The mermaids summon a storm and a pair of mermen mounted on giant spider crabs to attempt to rescue them. The mermen and their mounts are killed, but the crew suffer heavy casualties. The mermaids then summon a kraken, whose assault kills Abigail and more crew members and badly damages the ship. In the lazarette, Dahl reaches into the chest and retrieves a shell before he is killed by a quicksilver-like substance inside. Witterel enters the lazarette and kills two of the mermaids before the third finally sends the kraken away.

The third mate Martin Perrott and some men enter the lazarette later to set the last mermaid free. The mermaid instinctively spikes Perrott, who in his dying moments asks her to see the Obra Dinn home in exchange for her freedom. The others return the shell to the mermaid before putting her back in the sea.

Evans decides to abandon the Obra Dinn and make for Africa. Before leaving, Evans ties a rope to his monkey's wrist and sends it into the locked lazarette. He shoots the monkey dead and uses the rope to retrieve its paw, intending to use the Mortem on it later. Evans escapes with some others in a jollyboat, and a series of scuffles reduces the Obra Dinn's crew to four, including Witterel. The three others mutiny, seeking the shells, but Witterel states that he has thrown them into the sea and kills the mutineers in combat. Now the only one left, Witterel commits suicide near the body of his wife.

Unable to access the lazarette, the inspector catalogues the fates of 58 of the 60 souls aboard and leaves before a storm sinks the Obra Dinn. The inspector writes an insurance report, compensating or fining the estates of lost crewmen and passengers depending on their conduct, and mails the logbook back to Evans. A year later, Jane Bird—one of the three survivors who escaped with Evans—mails back the book alongside the monkey's paw and a letter explaining that Evans, now dead of an illness, requested that the inspector be given the means to complete the book as an expression of his gratitude. The inspector uses the Mortem on the monkey's paw to determine what happened in the lazarette and catalogue the last two fates, completing the story of the Obra Dinn for their collection.

Alternative endings

Bird reports Evans's death in every ending, but she only sends the book and the monkey's paw if the inspector determined all available fates. Otherwise, she will report that Evans appreciated the investigator's attempt or regretted trusting the investigator, depending on how many fates were determined.