The short version from IMBB:
The mythical world starts a rebellion against humanity in order to rule the Earth, so Hellboy and his team must save the world from the rebellious creatures.
The longer version from IGN:
After an ancient truce existing between humankind and the invisible realm of the fantastic is broken, hell on Earth is ready to erupt. A ruthless leader who treads the world above and the one below defies his bloodline and awakens an unstoppable army of creatures. Now, it’s up to the planet’s toughest, roughest superhero to battle the merciless dictator and his marauders. He may be red. He may be horned. He may be misunderstood. But when you need the job done right, it’s time to call in Hellboy (Ron Perlman).
Saving the world is a HELL of a job!
Download HD version at IGN.