http://www.webmd.com/sex/birth-control/features/plan-b-11-questions-11-answers
5. How does Plan B One-Step work?
Plan B One-Step works like other birth control pills to prevent pregnancy. The drug acts primarily by stopping the release of an egg from the ovary. It may prevent a sperm from fertilizing the egg.
If fertilization does occur, Plan B One-Step may prevent a fertilized egg from attaching to the womb. If a fertilized egg is implanted prior to taking Plan B One-Step, the drug will not work and pregnancy proceeds normally.
This is nothing I don't already know and I've addressed this several times in this thread.