Once during each City step, after you have visited all the locations you wish to visit and used all the services you wish to use for now, reveal the top card of the City Event deck, read the description on the card, and follow the instructions.
Most City Event cards will require you to choose one of several options on the card. If an option has a requirement, you must fulfill that requirement to choose that option. Your chosen option will usually direct you to read an entry in the Story Book.
Heroes may exchange items while resolving the card, but may not sell items, visit any locations, use any services, or retrieve stashed items unless instructed otherwise.
After resolving the card, remove it from play.
Heroes may then once again sell items, visit locations, use services, and retrieve stashed items until the end of the City step.