"Series" is lexically plural (it ends in -ies in English) but can actually be either singular or plural grammatically. So you have to say "It is a (television) series" rather than "it is series" or "they are series." Note that there is no such word as "sery" or "serie" as the "singular" form of "series." Again, "series" is both the singular and the plural form of the word. Think of Lemony Snicket's famous children's book, "A Series of Unfortunate Events."

Series can be a singular or a plural noun, depending on its meaning. When it is used to refer to a single set of things, it takes a singular verb even if it is followed by the preposition of and a plural noun: A series of medical tests is planned for next week. When series refers to two or more sets of things, it takes a plural verb: Three series of medical tests are planned for next week. Example in these phrase: "several series of books" "a series of books" . but in your sentence "They're series?" the word "they" refers to "outsourced materials" and is correct in plural form. The end

In English, we would say, "Have you seen Outsourced? It's a series." I am unsure why this is, it is just a word which you have to learn that despite there being an '-s', it is singular. To put 'series' in the plural, you would say, "Have you seen Outsourced and Friends? They're some series (on television)." (I added "on television" because it makes it sound better)

it's not "only plural" it's just like "deer" and "sheep", plural and singular firms looks same so you must say "It's a series".

When we use the word "series" in the singular, we are referring to a block or group of things as one. For example, The World Series is a group of games which determines the team champion. Yes, it's more than 1 game but it's the group of games that determines the winner. So instead of saying "The World Series" is airing on Friday. Another example is "Walking Dead" is a tv series. It's a show that tells the story over a period of time.