Complex sentence: Although the first film in the franchise was a remake of the Spanish film, "REC", "Quarantine 2" is unrelated to "REC 2", having a completely different plot and setting, while giving an alternative explanation for the infection.
Split into simpler sentences: Although the first film in the franchise was a remake of the Spanish film, "REC", "Quarantine 2" is not a remake "REC 2". Not only does the film have a completely different plot and setting, it also gives an alternative explanation for the infection.