What happens if i take plan b minutes after having unprotected sex?

Plan B: Hopefully it works and does it's job to prevent implantation of a fertilized egg to the wall of the uterus. But, that raises the significant question of why one would have Plan B ready before having sexual relations. Wouldn't it be better to just prevent pregnancy than to interfere with an impregnated egg implanting on the wall of the uterus?

Most likely it: will protect against pregnancy. Plan B One-Step is up to 89 % effective when taken within 72 hours (3 days) after unprotected sex. It can reduce chance of pregnancy by up to 95% if used within 24 hours. It continues to reduce the risk of pregnancy up to 120 hours (5 days) after unprotected sex, but is less effective as time passes.