Dr. Armstrong is a successful physician who comes to Soldier Island ostensibly to diagnose and treat Mrs. Owens for a condition he believes is likely invented. Armstrong thinks most of his patients need nothing more than a placebo medication because their issues are really ploys for attention. Armstrong attributes much of his success to his reputation. When his patients talk to their friends, they report that he quickly diagnoses and solves what other doctors cannot see. This encourages new patients to seek Armstrong out for similar solutions. He accepts Mrs. Owen’s case in part because she is connected to the widespread intrigue around the island, so he thinks the association will only help his name circulate in public circles.

Despite his insistence that hysteria is a woman’s condition, Dr. Armstrong is one of the first characters we see exhibit symptoms of psychological strain from the island. The first night, after Marston has died, Armstrong dreams he is operating on a patient whose identity shifts through different house guests with him on the island. As time passes, Armstrong becomes particularly unsettled by their circumstances and takes to smoking cigarettes to calm himself down. Stressed by the toll of the island, Armstrong finds his wits and nerves are unreliable.

The record accuses Dr. Armstrong of murdering Louisa Mary Cleese, one of his former patients who did not survive surgery. Initially, Armstrong seeks to distance himself from the situation entirely, claiming he doesn’t recognize the name. Later, he does acknowledge the failed surgery, but he is quick to jump to his own defense. Armstrong asserts it was a complicated surgery but does not reveal that he was inebriated at the time of the operation. He does show his competence as a doctor in diagnosing what chemicals are used to kill several of the characters, but he also initially dismisses Mrs. Rogers’ fears as merely the response of an anxious woman.

Armstrong relies on reputation to evaluate his peers, as well as to curate how they see him. He decides whom he can trust and whom he deeply distrusts based on their places in society. Rather than pay attention to them as individuals, he relies on what he expects from their roles in society. After the first two deaths by poisoning, Armstrong comes under strong suspicion from the rest of the house guests. As a doctor, he has easy access to and knowledge of poisons. However, Armstrong convinces the other characters of his innocence and points suspicion at characters he views as less respectable, and therefore less trustworthy, such as the Rogerses. Conversely, Dr. Armstrong implicitly trusts Judge Wargrave enough to form a secret alliance with him based only on the judge’s position as a respectable authority figure.  Armstrong helps Wargrave fake his death so the two can look for the killer with the advantage of surprise. Armstrong’s trust is ultimately ill-placed, and Wargrave uses their scheduled rendezvous to push Armstrong off a cliff where he drowns.