Who had been secretly following Helen on her visit to Holmes?
Helen Stoner is described as follows when Watson first sees her in the downstairs sitting-room at Baker Street:
A lady dressed in black and heavily veiled, who had been sitting in the window, rose as we entered.
Helen is naive and inexperienced in anything like deception and intrigue. She was afraid her stepfather Dr. Grimesby Roylott might follow her, and she tried to disguise herself by wearing what was evidently the mourning dress and dark veil she had worn at her sister Julia's funeral two years before. But she has only made herself all the more conspicuous. When Roylott discovers that his stepdaughter has left the house, he checks at the nearby Crown Inn and learns that Helen hired a dog-cart to take her to the train station. He takes a train to Waterloo Station in London and makes inquires among the cabbies. There was probably only one young woman traveling alone at that hour, and he is told that there was a young woman wearing a black dress and a heavy veil who arrived shortly before seven o'clock that morning. Roylott would have no trouble finding the cabbie who drove her to Baker Street. He hires the same cabbie to take him to the place where he left the girl off. Helen should have gone part of the way and then switched cabs, but she is inexperienced, frightened, and not at all cunning like her stepfather. When Roylott gets to Baker Street he makes inquires and finds that Sherlock Holmes lives at the house where the cabbie stopped.
Roylott is furious. He thinks he can scare Sherlock Holmes off by barging in and threatening him. He tells the detective:
"Don't you dare to meddle with my affairs. I know that Miss Stoner has been here. I traced her! I am a dangerous man to fall foul of! See here.” He stepped swiftly forward, seized the poker, and bent it into a curve with his huge brown hands.
“See that you keep yourself out of my grip,” he snarled, and hurling the twisted poker into the fireplace he strode out of the room.
Roylott probably does not expect Holmes to give him any trouble for two reasons. One is that he thinks he has frightened him off. More importantly, he knows that Helen doesn't have the money to pay Holmes a fee for his services. But he doesn't know that Holmes often works on a pro bono basis if a case interests him. In this case he is motivated both by his interest in the problem it presents and by his sympathy for the poor, frightened girl.
Dr. Roylott not only exhibits his tremendous strength but his superior intelligence. He is able to trace Helen from Stoke Moran to 221B Baker Street in such a short time that he arrives only minutes after his stepdaughter has left.