How to tell when to put article?

Question

I don't know when to put definite and when to put indefinite article so I have few questions.

"This is a true story about a man who chose the worst possible time for his crime. It happened in London in the summer of 1972. The man stole a barge on the River Thames (in case you don't know, a barge is a river boat used for carrying goods). The owner of the barge soon discovered that it was missing and immediately informed the police so that they could look for it."

I thought definitions are written with zero article like: in case you don't know, barge is a river boat used for carrying goods.(only barge not a barge)

Why is there the owner when we are only now introduced to him. Shouldn't there be an owner?

Why is there the police and not just police?

Answer 1

"The" is used for the owner because there are actions he takes part in.

"The" before the police is used because the report went to some actual police station or a group of policeman. In this case, the word "police" don't summarize all police.