It's not the plural form. It's the infinitive form. Other than that, you have the right idea.
It can be hard to see that difference. For most verbs, the plural form and the infinitive form are the same. One common verb with a distinct plural form, which we can use to see this difference, is "to be".
We can compare
Can John be a cat?
Can John and Mary be cats?
John is a cat.
John and Mary are cats.
Since the infinitive "be" is different than both the singular "is" and the plural "are", we can see that the "be" in the questions is independent of a singular or plural subject.
One of the rules for subject/verb agreement is that only the first word in the verb agrees with the subject.
John has a cat.
John and Mary have a cat.
Does John have a cat?
Do John and Mary have a cat?
When you put "does" or "do" in the front of the sentence, then that word becomes the first word of the verb. It is easy to notice that the singular "has" becomes the infinitive "have". It is not so easy to notice that the plural "have" becomes the infinitive "have", simply because those two forms of that word happen to look and sound exactly the same.
The verb "to be" is exceptionally irregular. Most verbs follow a simple pattern. The plural form is the same as the infinitive, and the singular form ends with an s.