The word "police" is rather special: It has no singular noun form. Something like that police over there is securing the scene would be incorrect. One would always construct sentences in the plural form like so:
The police are out in force today.
Anything done by the police will reflect on them.
Other words that take no singular form would include pants, trousers, scissors, and clothes.
Confusion arises because "police" is also used as an adjective. Consider these sentences:
A police department is housed in that building.
The police chief was highly visible at the town meeting.
In these two sentences, we are not speaking of "a police". You could easily remove the word from both sentences and they would make sense semantically and grammatically. Instead, the word describes the department or chief. It gives us context.
"Police" also has a verb form. You may encounter it like this:
The Boy Scout troop must police the area before they leave to remove any trash.
The verb means "to investigate, to search, to clean up". This certainly does fit in with a subset of the duties of a police department.
When you're using a countable noun in the plural, further references to that noun should honor the plural:
Dogs are the loveliest (not most lovely) animals in the world.
In the singular, you would say:
Dog is man's best friend. (example: http://mashable.com/2013/03/12/dog-mans-best-friend/)
The + countable nouns is tricky. You'd typically use it when talking about a specific instance of the countable noun, not in general. So "the dog" isn't appropriate, since we're not talking about a specific dog. There are some cases where you use "the", for example - the Moon, the Sun, the equator. All of these words have plurals, but of all moons and suns there are some of special significance to us, and so we use a definite article there.
Best Answer
There is some good guidance given by Oxford Dictionaries (OUP) here showing quite a number of examples.
The basic guidance is
So there are some that 'require' the -es although there isn't a hard and fast 'rule' that you can apply.
Consider zero and hero; indistinguishable in their form and yet zeros is the standard plural of zero while heroes is the standard plural of hero.
"Plurals of nouns". Oxford Dictionaries. Oxford University Press. http://www.oxforddictionaries.com/words/plurals-of-nouns (accessed September 21, 2014).