This is a plot summary for the Activision game Transformers: Devastation.
Plot
Decepticons arrive in New York City coming with a swarm of Insecticons and giant metal claws emerge from below, wreaking havoc. The Autobots: Optimus Prime, Sideswipe, Bumblebee, Wheeljack, and Grimlock arrive to investigate, battling Megatron and Devastator. Megatron escapes into a hatch leading underground and reveals the attacks are the automatic defenses of the Proudstar, an Autobot ship which crashed on Earth millions of years ago that Megatron now controls, and announces his plans to use the Insecticons to cyberform the Earth, turning it into metal and creating a new Cybertron.Optimus Prime explains that the Proudstar, captained by Nova Prime, was a ship meant to convert uninhabited planets into ones suitable to support Cybertronian life, and that its Ferrotaxis supercomputer contains a record of Cybertron's history and culture which had been lost during the Great War. Entering the ship, the Autobots fight off Soundwave and attempt to access the plasma core powering the Proudstar and the Insecticons, but the security system ejects the core from the ship. Returning back to the city, the Autobots obtain the core's location from Thundercracker after defeating him. Starscream challenges them to a fight hoping to win and change Megatron's opinion of him, but loses. As the Autobots get distracted by having to battle Blitzwing, Starscream arrives to take the core back to the Decepticons, but loses control of it due to a storm and crash-lands. The Autobots beat Motormaster to the core, but find it damaged and at risk of an extinction-level explosion. They manage to open the case but before they can shut it down, the rest of the Stunticons arrive and form Menasor, who loses to the Autobots but is able to throw the core to Megatron, who takes it to Cybertron to be properly repaired.
The Autobots follow the Decepticons through the Space Bridge, but are delayed by Starscream, Blitzwing and Shockwave, arriving too late to catch Megatron. Conversing with Shockwave, Optimus discovers that during the Proudstar's voyage, Nova Prime and the crew were corrupted by Unicron and had begun to cyberform inhabited planets as well, planning to do the same to Earth before they crashed. Returning to the Proudstar, the Autobots try once again to disable the core, but it ruptures and becomes inaccessible, leaving the Ferrotaxis, now in Megatron's possession, as the only way to stop the cyberforming process and keep the plasma core from exploding. The group head for the Proudstar's control room, fighting off the Decepticon forces as they ascend. Megatron escapes, ordering Devastator and Menasor to finish them, but the Autobots defeat them and follow Megatron, fighting him for the Ferrotaxis.
As he is defeated, Megatron fires the Ferrotaxis from his cannon into orbit and a swarm of Insecticons chase it. Optimus rides an Insecticon into space after it, with Megatron in pursuit. The two engage in a final battle above the Earth, with Optimus emerging victorious, but cannot shut down the Insecticons due to Megatron's tampering. Left with no choice, Optimus destroys the Ferrotaxis, stopping the cyberforming and preventing the plasma core's detonation. As he falls back to Earth, Optimus declares Earth is now their true home, and they must protect it at all costs. Later, with Defensor and Superion's support, Optimus commissions plans for the Optimus Maximus project.
In a post-credits scene, aboard the Proudstar, Nova Prime is seen awakening from stasis.
