Plan B is a contraceptive that prevents fertilization (as does the pill, an IUD, a condom etc..)
You are confusing Plan B with RU-486 that kills a fertilized egg.
https://www.nytimes.com/2012/06/06/health/research/morning-after-pills-dont-block-implantation-science-suggests.html?pagewanted=all&_r=0
".....Studies have not established that emergency contraceptive pills prevent fertilized eggs from implanting in the womb, leading scientists say. Rather, the pills delay ovulation, the release of eggs from ovaries that occurs before eggs are fertilized, and some pills also thicken cervical mucus so sperm have trouble swimming...."