https://www.fda.gov/drugs/postmarket-drug-safety-information-patients-and-providers/fdas-decision-regarding-plan-b-questions-and-answers
3. How does Plan B work?
Plan B works like other birth control pills to prevent pregnancy. Plan B acts primarily by stopping the release of an egg from the ovary (ovulation). It may prevent the union of sperm and egg (fertilization).
If fertilization does occur, Plan B may prevent a fertilized egg from attaching to the womb (implantation). If a fertilized egg is implanted prior to taking Plan B, Plan B will not work.