This is the 952nd most frequent Russian word.
сериала
The Russian word "сериала" means "of the series" or "of the TV show" (genitive singular of "сериал").
In this sentence, 'сериала' again takes the genitive singular form, showing the relationship between the 'plot' and the 'series'.
Мы обсуждали сюжет сериала весь вечер.
We discussed the plot of the series all evening.
The word 'сериала' in the genitive singular form shows modification, describing 'popular series' as the object of admiration.
Они являются фанатами этого популярного сериала.
They are fans of this popular series.
Here 'сериала' is in its genitive singular form, used to indicate possession or relation (of the series).