You should never ever ever tell someone the plot of your novel unless they're an agent or editor and have just asked. (And even then it shouldn't be a blow by blow description but a pitch - but that's for another post.)
Why the sweeping embargo?
A friend of mine confessed that after their partner left them, they kept telling people they met their story. Strangers at the bus stop, friends at parties, family get-togethers...it didn't matter who or where. After a while they started to notice people's eyes glazing over, and a little bit later they realised they were boring themselves, let alone others. They stopped having the need to tell their story (which was probably a good thing for them - and those around them).
Story tellers want to communicate to others. More than that, they need to communicate. Until they've communicated their story, they can't rest. But once they've told their story enough times, the desire goes. As a writer, you are a story teller. But the form of communication you're using is words on a page, not speech.
Writing a novel takes a lot of time and dedication. You're writing 80,000-100,000 words - that's a serious commitment. If you tell your story to people, you dissipate the desire to tell your story on the page. Eventually you may even become bored with the story you were telling, at which point you will stop writing altogether.
So keep it to yourself. If friends and family ask, simply tell them you'd rather not say. If they persist, you could always say that you're hoping they're going to buy a copy when it comes out, so you don't want them to know the plot before. Remember - tell no one...