Complex sentence: Cortex Strikes Back is a platform game in which the player controls Crash Bandicoot, who must gather 25 crystals for his nemesis Doctor Neo Cortex.
Split into simpler sentences: Cortex Strikes Back is a platform game in which the player character is the titular Crash Bandicoot. The goal of the game is to gather 25 crystals for Crash's nemesis Doctor Neo Cortex.