The subscribed version of the game has four cities (only New York is available to unsub'ed players).
In London, you can offer to retrieve an overdue library book, which can be in any of the four cities.
In Delhi, you can offer to deliver a package - again - it can go to any of the four cities.
Retrieving books and delivering packages are missions, and you will get a reminder on your detective page.
btw, you get 'paid' for completing either mission, and you can only hold one of each at a time.