Complex sentence: Although it is never officially stated in the cinematic release of "Revenge of the Sith" that Sidious was Plagueis' apprentice, it is stated on the official Star Wars Website that Darth Sidious was the apprentice of Darth Plagueis.
Split into simpler sentences: Although it is never officially stated in the cinematic release of "Revenge of the Sith" that Sidious was Plagueis' apprentice. It is stated on the official Star Wars Website that Darth Sidious was the apprentice of Darth Plagueis.