Your heroine is working late at the pharmacy when an armed gunman comes in and demands that she give him a very specific drug. In addition to the strange drug request, she notices something that makes her take extra care in the situation.
What does she notice? For example, can she see a small child waiting by the front door of the pharmacy. Is there another customer in the store?
What happens during and after the robbery? Why does the gunman want the drugs and what does he ask for?
Keep in mind that this story line could take place in the past, in the future, or on a distant planet. The thief could be a paranormal creature that needs a particular type of medicine to stay alive or keep the small child alive. Let your imagination go.