Helen Hudson mourns the sudden death of her husband, who drowned because the hospital's only pulmotor was being used on Robert Merrick, a selfish and spoiled youth who was drunk at the time he nearly drowned.  Dr. Hudson was a brilliant surgeon and philanthropist, and Helen is surprised to find that he had given away large sums of money during his lifetime.

Several people, including a sculptor named Randolph, reveal that they were the beneficiaries of Dr. Hudson's money, and that he helped a great many people in secret.  Robert falls in love with Helen at first sight, but his efforts are fruitless, as she holds him responsible for her husband's death.

One night Robert gets drunk and is given refuge by Randolph, who informs him that Dr. Hudson taught him how to "make contact with a source of infinite power," and that by making use of this information, Randolph elevated himself from a simple stonecutter to a sculptor.  Dr. Hudson's secret, which Randolph reveals to Robert, is to give help to people in utter secrecy, and never take back anything.  Robert takes this information lightly but later, gives a panhandler some cash and then sees Helen and believes she is his reward.

He insists she allow him to take her home, but when he runs out of gas and becomes amorous, she gets out and is hit by another car. Helen recovers, but loses her sight due to brain damage.  Robert watches her progress closely and befriends her, calling himself Dr. Robert so she will not know who he is.  When he finds out that her stocks and bonds are worthless, he secretly has them replaced with some of his own and then investigates the possibility of a cure through specialists.

Helen's sister-in-law, Joyce, and her friend, Nancy Ashford, meet Robert, but keep his identity a secret.  When Helen is approached by several eye specialists, she attributes it to her husband's renown, unaware of Robert's interest.

Helen travels to Paris to consult with the eye specialists, where she finds out that her blindness is incurable and falls into a deep depression.  Robert's arrival cheers her immeasurably, and after he proposes to her and confesses his true identity, she forgives him for everything and says he will receive his answer in the morning.  By morning, Helen has disappeared, leaving a note that she is afraid of being a burden to everyone and intends to take up life on her own.

Six years later, Robert returns to America a Nobel-Prize winning brain surgeon, having completed his medical education.  He is greeted by Helen's family and friends, who have not seen her in many years, and then by Randolph, whom Robert recalls as the man who taught him about Dr. Hudson's "magnificent obsession."  Randolph tells him that Helen is urgently in need of surgery, and Robert departs immediately and performs the operation.  The surgery is a success and, when Helen awakens, she has her beloved Robert at her side and her sight recovered.