According to the game's producer, knowing the Metal Gear Solid back story will not be required to understand MGS: Rising.
Don't expect to see a whole host of familiar faces in Metal Gear Solid: Rising, as Kojima Productions is planning to keep the number of cameos to a minimum to make the game more accessible to those new to the Metal Gear Solid series.
MGS: Rising is set after MGS2 and shows how Raiden became the cyborg ninja we wee in MGS4, but producer Shigenobu Matsuyama says that Kojima Productions doesn't want players to feel like they need to play the entire Metal Gear Solid series just to know who everyone is and follow what's going on.
"We don't really want to refer to and dwell upon the Metal Gear saga so much," he said. "We want newcomers to the game to come in and play freely without needing to know and understand the background of the whole Metal Gear Solid universe."
Matsuyama has said in the past that Rising is considered the start of a new branch of the Metal Gear Solid series, so it makes sense that Kojima Productions is trying to keep it as accessible as possible. But as the MGS series has one of the most complicated story in videogame history, Rising will probably still probably have plenty of twists and turns. I mean, it wouldn't be Metal Gear Solid otherwise, right?
Source: Connected Consoles