Don't be fooled by the 's' at the end. The singular and plural form of the word is 'series.' 'Serie' is not a word in English. You're referring to only one TV series about 'The Beatles,' not multiple, hence it takes a singular verb.

Paul is right. Series is both singular and plural. Although it is used to talk about multiple TV episodes, when those episodes are grouped together, that becomes a singular TV Series, a single succession or list of episodes.

it's plural not singular