Plan B works like a birth control pill to prevent pregnancy mainly by stopping the release of an egg from the ovary. It is possible that Plan B 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), which usually occurs beginning 7 days after release of an egg from the ovary. Plan B will not do anything to a fertilized egg already attached to the uterus. The pregnancy will continue.
http://www.fda.gov/Drugs/DrugSafety/PostmarketDrugSafetyInformationforPatientsandProviders/ucm109783.htm(Accessed May 23, 2011)
If an ovum is in the Fallopian tube, the process of fertilization may begin within 15 to 30 minutes after intercourse. The "morning after" is already too late for any contraceptive effect to intervene. Thus some researchers conclude that "post-coital drugs act principally to terminate a viable pregnancy by interfering with the endometrium: ... ‘this mode of action could explain the majority of cases where pregnancies are prevented by the morning-after pill.'" (Wilks, op. cit., p. 154, citing Grou, F. and I. Rodriges. "The morning-after pill; How long after?" Am. J. Obstet. Gynecol. 171:1529-34 (1994).)