Prior to the beginning of the game, Caim's parents were killed by a black dragon sent by the Empire, breeding a hatred in him toward both the Empire and dragons. During the war against the Empire, Caim is mortally wounded, but comes across a red dragon imprisoned by the Empire. Although neither Caim nor the dragon are fond of each other, they cannot deny that they need each other's help to survive and become pact partners. The price Caim paid to enter the pact was his voice, rendering him mute and causing a pact emblem to appear on his tongue.
Together, Caim and the dragon fight against the Empire with the help of allies they meet along the way, including other humans, elves, and other pact creatures. Together, they eventually discover that the Empire is led by a six-year-old girl named Manah, high priestess of the Cult of Watchers. Having been driven insane, the young girl has been possessed by otherworldly entities that desire the Goddess and the seals that prevent chaos from overwhelming the world and destroying it. In the end, Caim and the dragon manage to stop them, though the dragon, whose name is revealed to be Angelus at the game's end, volunteers to become the new seal and disappears. This is one of five endings and is the one which leads to the events of Drakengard 2.
The second ending involves Inuart and Furiae after she is killed. Inuart takes Furiae and tries to resurrect her through evil. Although she is revived she is a monster, now having godly powers and wings. She kills Inuart and Caim must now kill his own sister. After doing so you see that although he has killed his own sister, the world is now filled with these creatures that look like Furiae and all is lost.
The third ending contains Caim and Angelus. After Manah sees that things are lost for the empire she tries to summon a dragon but the dragon eats her. All her evil power goes into Angelus, and she now becomes the Chaos Dragon. After becoming this new dragon the pact between Caim and Angelus is broken. But Angelus, although still having respect and love towards Caim feels that if he is not killed all of humanity will be lost. After this fight Caim pulls through and defeats her but after killing her he hears more dragons coming his way. Happy for more killing and fighting you see Caim run outside full speed to battle until his final breath.
The fourth ending consists of Caim, Angelus and Seere. After Seere had to kill his own sister, Manah, the world comes crashing down and all order is lost. Giant babies, also called Grotesqueries, come falling from the sky. They are eating and destroying everything in sight. Arioch sees all these children and becomes obsessed with them. She run towards them and they eat her. Then Leonard sacrifices himself by blowing himself up to clear a path for Angelus, Caim and Seere. After doing this, the Queen Grotesqueries, which looks like a giant woman with no hair, comes up from the ground. Angelus and Caim know that their world is lost without a miracle when they realize what to do. Seere, whose sacrifice for a pact was time, has the ability to freeze time, but he must sacrifice himself. Caim, Angelus and Seere fly towards the Queen Grotesqueries and let Seere drop on top of her. Angelus is then bitten by the Grotesqueries and you see Caim's sword fly through the air as if to say they are both dead. Seere asks for Manah to forgive him and send bursts of light out from him. You then see that the area with the Queen Grotesqueries and all her children are covered in a thick, black fume and time is frozen in the black gas.
In the fifth ending, Caim and Angelus travel across the dimensional boundary to fight the grotesqueries, and in a surprising twist end up flying over modern Tokyo. After defeating the grotesqueries, they are both killed by a fighter pilot that shoots them out of the sky.