An open letter, stuffed in a bottle and apparently tossed into the sea, is recovered and sent to Scotland Yard. The letter is signed by Lawrence Wargrave and offers Wargrave's confession to the murders on Indian Island.
In the letter Wargrave speaks of an early fascination with two significant issues: death and justice. He discusses how he fed his appetite for the two by becoming a judge and notes that at some point in life he determined that he wanted to commit murder on a grand scale.
He discusses how he devised a plan to locate individuals guilty of murder, as well as how he set up the murders on the island. He reveals that he took Armstrong as an ally so that he might fake his own death, thereby making the final four murders less detectable.
Wargrave states that the order of the death was based on the guests' "varying degrees of guilt". He also admits to taking his own life when all other guests had been killed. The letter ends with Wargrave noting that he had wanted the plot to be a perfect crime, but in the end decided to flaunt his success through the letter.
The final chapter offers a highly plausible solution to the Indian Island murders, a solution that makes the characters' inability to form true alliances and to effectively interpret the mystery's clues much more poignant. The conclusion also calls into question the whole notion of crime, justice, and punishment. How many criminals get away with their offenses? Did Wargrave have the right to take justice into his own hands? Was the punishment of death appropriate for each guest? Of course, there is always the possibility that Wargrave's letter is the true red herring mentioned in the nursery rhyme.