Plan B is not as effective as regular hormonal birth control. It's simply worth a try in an emergency, & has more chance of being effective, the sooner you take it.
Plan B may be the wrong choice of emergency contraceptive, if you are around your time of ovulation.
Plan B works by delaying your ovulation until the sperm leaves your body. If you're already ovulating then it can't do that.
In that situation you'd need one of the other methods of emergency contraceptive; either the other emergency pill called Ulipristal Acetate (Ella), or to have an IUD fitted by a doctor, within 5 days/120 hours of the unprotected sex.
Day 1 of your menstrual cycle is the first day of your period.
Most women who have regular periods ovulate somewhere around days 12 to 18 of their menstrual cycle. That's a safe estimate. With a 28 day cycle it's typically more like day 13-16.
In the week leading up to ovulation, cervical mucus changes in consistency, so that it can save any sperm that enters your vagina, ready for when you ovulate (release an egg).
That means that you can get pregnant from sex you had up to a week before ovulation.
If you were ovulating, or close to it, you'd need one of the other methods of emergency contraceptive, mentioned above.
So it very much depends on where you were in your menstrual cycle, as to whether Plan B might work for you.