Wise men discovered once another world: the Outworld.
In the Outworld they have a leader: Shao Kahn.
Shao Kahn sent Shang Tsung to earth to make earth a chaos.
3 warriors from earth search under aid of Rayden (the Thundergod) other warriors to protect the earth.
Shang Tsung invites all warriors to compete in his tournament. He hostages Sonya and Kano when they ran into him. Shang Tsung and his helper Goro, a half human, half dragon, die in the tournament. In the Outworld Shang Tsung appears to Shao Kahn (in the Outworld the dead can be brought to life). He suggests to let the earth warriors to see certain death by Shao Kahn himself. Shao Kahn agrees and restores Tsung's youth again.
Shao Kahn gets  some help of Kintaro and does not get killed.
In Ultimate MK3 he appears again, because his died wife Sindel has come alive again on EARTH. He opens the gates from the Outworld to earth again.
Rayden and his team protects the earth and Shao Kahn gets defeated. But then Sub-Zero frees Shinnok by accident. Read that in Mortal Kombat Mythologies.
Shinnok is an old school god who misbehaved in the past and was locked up in the Netherrealm by Rayden.
Shinnok is now back and grabs the power. The sorcerer that gave orders to Sub-Zero to free Shinnok, becomes Shinnok's helper.
It's now up to you to save earth in MK4.
Have a nice day.