The sailor answers Dupin's Ourang-Outang-Found ad in Le Monde, a newspaper popular with seafaring guys.
He offers Dupin a small reward for finding the animal and reporting it.
Dupin says, sure, that's fine, and by the way, what do you know about the murders in the Rue Morgue?
The sailor is horrified, but decides to confess all.
He has recently visited Borneo, where a pal of his helped him capture an Ourang-Outang that he brings back to France to sell.
The friend dies and the sailor has sole custody of the ape.
On the night of the murder, the sailor goes out with a bunch of friends and leaves the Ourang-Outang locked in his closet.
He comes back and is horrified to discover that the big ape has gotten out of the closet and is mimicking the sailor's shaving routine with his razor.
The sailor goes for his whip to punish the animal, but the Ourang-Outang is too quick for him, leaping out the open window for freedom.
The sailor chases after the beast.
He watches the ape swing up through the open window on the fourth floor of the house in the Rue Morgue – the ape is attracted by the light.
The sailor climbs up the lightning rod next to the building and swings over to the window on an open shutter.
He watches, unable to do anything, as the ape slashes the throat of Madame L'Espanaye with his razor and then strangles Mademoiselle L'Espanaye to death with its bare hands.
The Ourang-Outang sees the sailor through the window, gets frightened, and tries to hide the bodies by putting Mademoiselle L'Espanaye up the chimney and picking up Madame L'Espanaye to throw out the window.
Frightened by his ape coming towards the window, corpse in hand, the sailor flees down the lightning rod and runs away.
Later, after he tells all this to Dupin, he manages to catch his Ourang-Outang again (we're not told how).
He sells the beast to a zoo – so, all's well that ends well?