Do I have to tell the truth in my query? I mean about the plot.
Yes, but you don't have to tell the whole truth and nothing but the truth. If one sentence of truth is going to necessitate three paragraphs of explanation, better to tell a half-truth. For instance, say your hook is:
You might want to avoid explaining all the extenuating circumstances that make this a reasonable decision by softening the hook to:
Ellen's ex-con boyfriend has tried to kill her six times in the past two weeks, but now she's considering accepting his marriage proposal.
Her hunky guy isn't exactly a model citizen, but Ellen's always been attracted to the roguish type. Can she finally tame this bad boy?
If your query states that the hero and heroine of your romance novel live happily ever after, and the editor reads your manuscript only to discover that the hero ditches the heroine on the eve of their wedding and marries her despicable rival, you won't be getting any future manuscript requests.
If your thirty-year-old main character does something so stupid that even a five-year-old would know better, you have two choices in your query:
1. Don't mention it at all.
2. Declare that your main character is four years old.
I recommend #1. A lie of omission is better than a bare-faced lie that will be exposed on page 1.