Not really. It's meant to prevent the release of an egg from the womb (ovulation), so if it already happened, you're unlikely to be protected. From its website:
"It works mainly by stopping the release of an egg from the ovary. It is possible that Plan B One-StepĀ® may also work by preventing fertilization of an egg (the uniting of sperm with the egg) or by preventing attachment (implantation) to the uterus (womb)."