Susanoo is said to have insulted his sister Amaterasu by saying she had no real power over the high realm.
After this, his father, Izanagi, ordered him to leave. Before leaving he went to bid his sister farewell, but Amaterasu was suspicious of his intentios.
To prove his sincerity, Susanoo proposed a challenge in which both Gods had to take a possesion of the other and turn them into humans.
Since Amaterasu's necklace birthed more people than Susanoo's sword, she was claimed the winner. After his defeat, Susanoo went on a rampage, and killed one of Amaterasu's attendants.
This action affected the advancement of the plot because it led Amaterasu to retrieve to a cave, which cause the world to plung into darkness and chaos.