Complex sentence: It is the sequel to 2002's "Godzilla Against Mechagodzilla"; the only film in the Millennium "Godzilla" series to be a sequel to another film in the same series.
Split into simpler sentences: It is the sequel to 2002's "Godzilla Against Mechagodzilla". This is the only film in the Millennium "Godzilla" series to be a sequel to another film in the same series.