I am certainly not an endocrinologist or an expert on Plan B, but I did spend under a minute Googling:
When you purchase Plan B, you get two pills. Each pill contains 0.75 milligrams of levonorgestrel. You take the first pill as soon as possible after unprotected sex, and you take the second pill 12 hours later.
Although scientists aren't completely sure how it works, they believe that levonorgestrel prevents pregnancy either by stopping the ovulation process or by disrupting the ability of sperm and egg to meet in the fallopian tubes. Some speculate that the drug may prevent the fertilized egg from implanting as well, perhaps by making the uterine lining less receptive to the egg.
Levonorgestrel does this by disrupting the natural hormonal cycle. It contains a synthetic form of progesterone (regular birth-control pills contain it in lower doses). The high doses of progesterone in Plan B are disruptive enough to prevent fertilization or implantation.
Discovery Health "Plan B"
How, again, does Plan B not interfere with female hormones? :lamo