After the deaths of Anthony Marston and Mrs. Rogers, Philip Lombard discusses the situation with Dr. Armstrong. They arrive at a theory that the mysterious U. N. Owen who brought them all to the island is hiding somewhere on it and murdering them one by one. However, Lombard observes, Indian Island is "more or less a bare rock," which should make it easy to find Owen if they search it thoroughly. Dr. Armstrong is concerned that any man who could formulate such a murderous plan must be "a raving maniac" but agrees to help in the search, and they ask Mr. Blore to help them. They reject the idea of including Mr. Justice Wargrave or General Macarthur in their search party, on the grounds of age and possible senility.

The search for U. N. Owen is a classic example of one of Agatha Christie's red herrings, but the seasoned reader of Golden Age detective fiction already knows that the search party will not find any such person. The Golden Age detective story is an intellectual puzzle which has nothing to do with realism. The idea that the killer could be some lunatic of whom the reader knows nothing but who is found hiding in a cave somewhere is too inartistic to be considered.