As she came nearer, her dress showed her to be a servant.

She had a large door-key in her hand, and was evidently going out to perform an errand.

Roused partly by curiosity, partly by the impulse of the moment, which urged her impetuous nature into action after the passive endurance of many hours past, Magdalen snatched up her bonnet, and determined to follow the servant to her destination, wherever it might be.
The woman led her to the great thoroughfare of shops close at hand, called Lambeth Walk.

After proceeding some little distance, and looking about her with the hesitation of a person not well acquainted with the neighborhood, the servant crossed the road and entered a stationer's shop.

Magdalen crossed the road after her and followed her in.
The inevitable delay in entering the shop under these circumstances made Magdalen too late to hear what the woman asked for.