Learn Spanish (Latin America): Nos interesan mucho las series de misterio.

This verb works exactly like gustar. The actual subject of the Spanish sentence is las series de misterio (the mystery series), which is plural. Literally, the sentence says "The mystery series are interesting to us." Since the series are doing the interesting, the verb takes the plural form interesan.

Why does the sentence use mucho instead of muchas, since las series is feminine plural?

Here, mucho is acting as an adverb (a word that modifies a verb). It tells us how much they interest us, rather than describing the series themselves. When mucho modifies a verb, it always stays exactly like this and never changes its ending to match gender or number.

Singular or plural: Is series always spelled with an "s" at the end in Spanish?

In Spanish, the singular is la serie and the plural is las series. Because we're talking about multiple series in this sentence, we use the plural las series. Notice how this also forces the verb to be the plural interesan instead of the singular interesa!

Why do we say series de misterio instead of misterio series?