A young man returns to the lighthouse, where his father is in charge, with the hope of marrying his adopted sister. During his absence, however, she had fallen in love with the captain of a schooner. Her only companion on the island was a dog. This dog is bitten by another while it is on the mainland and in turn the dog bites the rejected suitor. The father and adopted sister, both unable to read, do not realize that the young man is sick. The dog has been sent to the mainland for examination. The son receives a letter to the effect that the dog is affected and anyone bitten by it should come for treatment. This letter is torn up. The sister and father piece it and take it with them on their visit to the mainland to secure provisions. They are told of the young man's danger. The victim has begun to feel the first effects of the disease, though he had laughed off any possibility of his getting it. Night creeps on and with it a fog. The sea captain, returning with his aged parents for the wedding on the island, finds himself in danger when the light fails to appear in its customary place. The old father and girl, after much difficulty, arrive at the island; the father is forced to struggle with his own son to save his own life. When the son is accidentally killed, the old man is heartbroken. He rebels against fate, crying, "I have tended this light for forty years and now my boy is dying," and his interest in the darkened light above that is to save lives, slackens. The girl has climbed to the tower, but is unable to master the difficult lights. The ship carrying her sweetheart is almost upon the rocks. She calls the old man. He hears the call and he finds that the cry of duty is stronger in his breast even than any other and he responds.
—Moving Picture World synopsis