That would be the 'Guns of the Patriots' one. It will explain to us how Raiden became a cyborg like ninja badass after MGS2: Sons of Liberty.
A trailer recently went up at E3 that showed Raiden and his slice-and-dice blade of uberness carve up a mech, and then rip out a blue glowing spinal cord thingy and absorb its energy into his suit. This will be the way the game progresses apparently.
Director Minesh Kimura says there will still be some "stealth elements" to MGS: Rising despite its mostly hack and slash action. Raiden won't just have the blade but some sub-weapons to clean the field of foes too.
More story details are expected soon. Metal Gear Solid: Rising releases on Xbox 360, PS3 and PC although no release date yet exists for the instalment. Does this more action orientated Metal Gear elicit your gaming interest?