In the heart of the Sumatran jungle, Doctor Paul Hallet and chemist Jack Murray, working to develop a vaccine for red fever, bury their colleague Adams when he contracts the fatal disease. Park Avenue doctor, Phil Saunders, the nephew of a veteran fieldworker, is sent to take Adams' place. Hallet, expecting the uncle, ostracizes Phil and relegates him to washing test tubes. One day, "Junior," a baby monkey, cuts himself on a test tube and drips blood onto a live culture slide, and the blood kills the diseased cells. Further tests prove that Junior is immune to red fever. Eager to prove his theories, Phil secretly conducts experiments on baby monkeys believing they possess the key to the vaccine, while Hallet and Murray run tests on their mothers, who have been injected with Hallet's serum. On the day Hallet is sure he has found a vaccine, Phil's theory proves correct, but he defers to Hallet's expertise and remains silent about his own success. Hallet is about to be a guinea pig for his own antitoxin, when Phil announces he injected himself with it. As Phil lies in a feverish state, Hallet receives word that their operation is being closed down, and the mother monkeys whom he thought were cured die. Delirious, Phil confesses he made his own serum and tells Hallet to read his lab notes, then dies. While packing to leave, Hallet discovers Phil's notes and $4,000 in traveler's checks and decides to pose as Phil and carry on his research so that Phil might get credit for the discovery of the vaccine. The newspapers announce Hallet's death and Phil's groundbreaking research, and a new assistant, Dr. Mary Reynolds, arrives. Again assuming his new assistant is incapable, Hallet is surprised to learn she conducted fieldwork for six years. Meanwhile, Phil's self-centered society wife, Claire, in Paris for a divorce, reads of Phil's chance at the Nobel Prize and drops her divorce plans in favor of publicity. Back in Sumatra, Mary and Hallet fall silently in love, but he refuses to get close to her and asks her to return to Singapore. The next morning, Mary is about to leave when Claire arrives by private plane. Hallet is forced to confess his true identity and Phil's death four months before. Claire vows to prosecute Hallet for murder and forging Phil's signature, but Murray has her pilot leave without her. Claire then contracts red fever, and Hallet is forced to innoculate her with Phil's just-completed serum. Hallet saves Claire's life, but she is still resigned to prosecute him, until Murray appeals to her suspicious nature and convinces her that prosecuting Hallet is just the sort of publicity he is after. She leaves haughtily, vowing to publish her husband's notes and establish his place in medical history to spite Hallet. Mary and Hallet look forward to working together to fight another epidemic.