Story

Orochi, the Serpent King, created a rift in time and space. He created a twisted new world and brought together warriors from the Three Kingdoms era of China and the Warring States period of Japan (more than 1,200 years apart in history). Orochi wished to test the might of the warriors of these two eras.

The story is told in four separate, unrelated subplots. Each subplot starts the player with three characters. More characters are unlocked as the player progresses through the story or satisfies certain conditions in certain stages. Though each separate subplot is named after the Three Kingdoms (and one from the perspective of the Samurai Warriors characters), characters from different factions band together as one in each subplot to confront Orochi in an ultimate showdown. Due to the storyline, a majority of the characters have split from their respective factions in their original games and have been forced into others. However, the character selection screen still places all the characters in their original positions

-Wikipedia

Wu Storyline

In the Wu story, Sun Jian is captured in a battle with Orochi. Orochi uses him to blackmail Wu, more specifically the Sun Family, into service under him. However, Sun Ce, unable to bear the shame, rebels, earning him the hatred of his sister, Sun Shang Xiang, and brother, Sun Quan.

Shu Storyline

In the Shu story, Zhao Yun has been captured and imprisoned, having lost his will to live after his Lord Liu Bei's supposed death. However, when Zuo Ci, Xing Cai, and Yoshihiro Shimazu inform him of Liu Bei's survival, he and many new recruits embark on a perilous mission to locate and rescue their lord. 

Wei Storyline

Cao Pi has taken control of the army upon his father's disappearance, and decides to ally with Orochi. Many members of Wei flee his control, disgusted with his decision. However, as the story progresses, Cao Pi reveals his plot-he has been tearing Orochi down from the inside...

Sengoku Storline

Oda Nobunaga, the Demon King, despises Orochi and establishes his own army in order to slay the Serpent Lord. His old enemies, Takeda Shingen and Uesugi Kenshin, also fight to defeat Orochi, but none of the armies will comply to ally.

-Written by me