Hm? A cabal of AIs named after dead presidents gets infected by a virus written by Otacon's sister, and call you repeating corrupted nonsense (I need scissors! 61!) while you sneak naked through Arsenal Gear. Snake shows up and gives you your gear and a katana. You then fight 100 dudes, then have to fight 30 Metal Gear Rays on the deck of Arsenal Gear. AG crashes into Wall Street (they removed that cutscene because of 9/11), then you fight Snake's long-lost brother/clone who happens to be the former President of the United States and attacks you with mechanical octopus arms. The AIs call you and tell you that you performed perfectly, because the whole game was a simulation they devised to find the perfect Solid Snake alternative to infiltrate and kill the President, because the President was going to kill their plans to control the Internet and selectively bias information dispersal to control humanity and promote order, and Snake wasn't going along with their plans. Then you find your girlfriend in lower Manhattan where everyone is just walking around like a 3000-ton mechanized warship didn't just take out the entire neighborhood.
What's hard to understand?