Write a synopsis. Then get feedback on that.
Your synopsis should be as brief as possible, conveying only the elements that are absolutely crucial to the story - the elements without which the story would be absolutely different. If you were summarizing the first Harry Potter book, you don't need to say "and then he went to a Potions class, and then he went to a Quidditch game, and then...". Instead, it'd look something like this:
- Harry is an orphan sorely mistreated by his aunt and uncle.
- On his 11th birthday, he is suddenly summoned to Hogwarts, a wizarding school. His aunt and uncle try to stop it, but their attempts are in vain.
- Harry has a secret past: evil Voldemort killed his parents and disappeared; Harry is famous as "The Boy Who Lived."
- Harry has lots of exciting adventures at school as he learns about the secret wizarding world. One of his teachers is Snape, who is cruel; another is Quirrell, who is bumbling and pleasant.
- Bad things are happening at Hogwarts - attacks and destruction.
- Harry and his friends find out about the Philospher's Stone, which grants eternal life, and is hidden in Hogwarts. They realize Voldemort is after the stone. They think he is acting through cruel Snape.
- There is a break-in aimed at the stone. Harry and his friends race to reach it first, overcoming wizardly tests and challenges.
- It turns out the villain is Quirrel, who is harboring Voldemort. Snape has been trying to protect Harry all along; Harry's misunderstood him completely.
- The last line of defense makes sure the Stone will go only to someone who wants the Stone itself, rather then the long life and riches it can provide. Harry gets the stone and defeats Voldemort's current incarnation.
- Harry is sent back to Privet Drive for the summer.
Now, this is a very simple, straightforward plot, that skips almost all the "meat" of the book. That's fine. That's what you're after - just the absolute essentials.
Now your synopsis is critique-able. If you have any overarching problem that would destroy your story's credibility, it would be evident at this point. This plot is clear enough for questions like "How can he possibly write a virus for an alien computer system he's never seen?" or "Why doesn't she just call the cops?" or "He does all that just to get the girl? That sounds awfully stalker-y to me" or "This doesn't seem to have an actual ending" - narrative problems big enough to substantially affect your entire book, not just a couple of scenes. Since you've eliminated all the non-crucial details, it's safe to assume that as long as you stick to the outline, anything new that you introduce might still raise narrative problems - but they'll be local, a single character or chapter that needs to be fixed, rather than "this whole book makes no sense."
If necessary, your synopsis might touch on the cast of characters; you might include subplots, or have a seperate synopsis for each major plot thread. The important thing is to include everything that's crucial to the whole, and leave out anything that isn't.
As always, treat feedback with caution. Some reactions you'll be able to dismiss; others will point out weaknesses that you should take care to shore up; some might demolish your plans entirely. Figure out which is which before you get too worked up about any of them.