The Disaster -
Nova Prime (not that one) lands on Knowhere and steals the Aether, known to Cybertronians as the Darkness. Transforming into the planet sized Nemesis Prime he destroys the Celestial head, and makes his way toward Xandar. He senses the closest infinity stone, the next goal in his quest to wipe out all organics in the universe and recreate it in his image as part of the Expansion.
Team One - Transformers
On Cybertron, Optimus Prime feels the awakening pull of the Darkness across the cosmos. His diametric opposite Nova Prime has set in motion a plan that will spread the 'glory of Cybertonian culture' and destroy all things in the universe if he is not stopped. Optimus puts together a team of his most trusted comrades in arms to hunt down Nemesis Prime. With the help of Omega Supreme, the largest of the Autobots, as a transport they approach Xandar where they meet up with…
Team Two - Guardians of the Galaxy (MCU version)
Peter Quill receives a call from Nova Prime of the Nova Corps requesting back up from the Guardians. Hoping that Quill and his team's experience with the Orb will come in handy fighting the gigantic robot threatening to demolish Xandar and take the Orb of Power for itself. The Guardians race to the Xandarian home world just in time to see the arrival of the Autobot army. While establishing friend from foe, Autobots and Guardians team up to defeat the common enemy.
Optimus Prime and his band of Autobots combine to become a new form. They use this new form to blast holes in Nemesis Prime, allowing Quill's ship; the Milano, and Nova Corps ships to infiltrate the interior of the planetoid sized villain. Once inside the members of the Guardians race to find the area of the evil Prime containing the Darkness.
Emerging in a chamber of the dark red heart, Groot uses his abilities to wrap a ball of branches around the matrix containing the Aether. As they touch it they begin to expand and grow, choking out the power to Nemesis Prime. The robot has been disabled but not destroyed, and hangs like a mechanic moon over Xandar.
In order to destroy the lingering shell of the powered down Prime, the Autobots agree to let Rocket turn their combined form into an incendiary device. The combined bots target the core of the massive Prime. The Milano flies off with its new cargo as Nemesis Prime is overwhelmed by an all consuming fire ball.