Optimus Prime fights for truth, justice, and the Autobot way. He has led the search for the Mini-Cons, faced the resurrection of a dark god, and now aims to stop Megatron from obtaining the Cyber Planet Keys. He's also died a couple times, and tends to find a second-in-commandy-type to combine with to form a super robot.
Also, he wants to make sure you don't sit too close to the TV screen. 
Millions of years ago, a young Optimus Prime fought with a great Decepticon warrior known as Scavenger. So often were their clashes, and so familiar were they to each other, the pair eventually became friends, and Scavenger signed up with the Autobots. Seeing great potential in the young Autobot leader, Scavenger taught Optimus Prime how to fight in hand-to-hand combat. Later, veteran Autobot Landmine trained Optimus in metallikato shortly before leaving Cybertron with Rodimus. Optimus later notes that their training made him the warrior and soldier he is today.
Many years later, it is a more mature Optimus Prime that leads a small Autobot unit to Earth after the activation of the Mini-Con homing beacon. This beacon signaled to all the Transformers on Cybertron that the much-coveted Mini-Cons had survived their million years-long disappearance. After warping to Earth, Optimus saves three human children from the Decepticon leader, Megatron, who arrived on the alien world first.
Clashing with his arch enemy, Optimus Prime bought the children and their Mini-Con High Wire time to escape while around them, and other Transformers warped into the battlefield. Later, after the Decepticons had reformatted themselves into suitable new vehicle modes, Optimus Prime saves the children again, joined by his two subordinates Red Alert and Hot Shot.
Befriending these children, he introduces his crew of Autobots to them. The children aid Prime as he leads his crew around the world, searching for the long-lost Mini-Con Transformers, before the Decepticon forces can take them and use them for their own evil deeds.
Soon afterward, both the Autobots and Decepticons detect a Mini-Con signal located in the Grand Canyon. With the help of the kids, Optimus Prime manages to pinpoint the location of the Mini-Con panel before he is confronted by Megatron, powerlinked to Leader-1. Outgunned, Optimus Prime is saved when Starscream arrogantly underestimates the Street Action Team's combined form of Perceptor. This distraction gives Optimus the opportunity to activate the Mini-Con panel, revealing that its occupant is Sparkplug, Optimus Prime's old Mini-Con partner and friend. Together they Powerlink, activating Optimus' powerful Maximum Blast which sends the Decepticons into retreat.
Through the treachery of Sideways and Thrust, the Decepticons manage to secure all three of the legendary Mini-Con weapons that form the Hydra Cannon, the most powerful weapon in the universe. Mounting their new doomsday device aboard a starship, Megatron decides to wipe out the Autobot base on Earth, regardless of the massive collateral damage to the planet itself. Optimus Prime and Jetfire launch a pre-emptive strike against the warship in the hopes of disabling the weapon and rescuing the Mini-Cons. However, they are outnumbered, forcing a desperate Optimus to throw himself into the path of the cannon and use the Matrix of Leadership to save Earth, but at the cost of his life.
Under the leadership of Hot Shot, the other Autobots give chase to the Decepticons aboard the Axalon. During a confrontation with their enemies, the Mini-Cons reach out to each other and resurrect Optimus from within the Matrix. There is little time to rejoice, as the Autobots pursue the Decepticons on their way back to Cybertron.
After Unicron's apparent defeat, both Optimus Prime and Galvatron succumb to their base natures and have one last battle within the Chaos Bringer. Fighting each other to a standstill, both are surprised when Unicron suddenly comes back to life, feeding off their hate for each other. Galvatron is drawn toward Unicron's maw, with Prime holding onto his tank treads, telling him to hold on. His nemesis then makes the decision to sacrifice his life, giving Prime a final victory, so that Unicron may be destroyed and their race will survive.
In the aftermath, Optimus Prime reflects that he doesn't deserve the Matrix of Leadership and leaves both the Autobots and Cybertron for places unknown.
A few years after the defeat of Unicron, Optimus Prime returns from exile to a newly-unified Cybertron where Autobots and Decepticons live side-by-side in peace. Though he can no longer combine with his trailer, he now combine with the Prime Force. Reassuming leadership of the Autobots, Optimus Prime establishes a dialogue with the Earth governments with the help of his human friends. Earth sends a delegation to Cybertron, consisting of the three kids and Dr. Brian Jones, the foremost expert in the newly discovered ore energon that both worlds covet.
Jones had brought along his young son, Chad (also known as 'Kicker'), who was frightened by the tall, menacing robots. When the boy ran away to a ruined and abandoned part of Cybertron, Optimus Prime gave chase, warning Kicker that the area was strictly off limits. The boy accidentally fell and found his way into a massive darkened chamber where an energy being known as Primus saved him. Optimus Prime found his way into the chamber just as Primus evolved the human child. Sometime after this, Primus (now known as the creator-god of the Transformers), gave Optimus the Spark of Combination, allowing Optimus to grant combining powers to other Autobots.
Almost a decade later, it is brought to the attention of Optimus Prime that the energon mining facilities in Sol's asteroid belt, as well as their base on Mars, had been attacked by a mysterious force of beast-like Transformers. Primus warns him that the enemy is after the energon, causing Optimus to assemble a unit so he can personally assess the situation. Finding no life on Mars, his unit then moves to Earth, where he saves Kicker and Ocean City from an attack by the same creatures.
Primus' plan to trap Unicron inside an energon sun has failed, and this star has collapsed into a massive black hole. Optimus Prime faces the greatest test in his existence when his home planet is threatened with destruction, but realises there is no choice but to evacuate the population to Earth. As they make plans to leave, a mysterious time-travelling Transformer named Vector Prime appears, advising Optimus to search for the Cyber Planet Keys, the only means of restoring Primus' spark, and the only way they can save Cybertron and the rest of the universe from oblivion.
Arriving on Earth, Optimus Prime is forced to break his own strict orders to hide from the human population when he rescues the injured Landmine and three human children from Thundercracker. Befriending the children, named Coby, Bud and Lori, he relies on them to educate the Autobots about their new home and how to blend into the environment as Robots in Disguise.
Soon afterward, the children's hometown of Boulder, Colorado is threatened with devastation by a landslide. Understanding how the humans would feel at the loss of their home, the Autobots leap into action, rescuing people trapped and leading them to safety, all the while in disguise. Optimus Prime (somehow) prevents the landslide by pumping water out of an underground lake beneath the mountain, causing it to collapse in on itself and eliminate the threat.
Optimus Prime is notified by Hot Shot that the Decepticons are attacking Mini-Con villages. Later on, when the war comes to Earth, he saves the kids from Megatron, who is trying to get the Mini-Cons that they have befriended. After retrieving a Mini-Con escape pod in Alaska, he is suddenly pulled into an alternate reality Cybertron, one in which Unicron has already decimated the planet and is preparing to return and consume it. He is ultimately rescued by Jetfire and Over-Run, and uses his knowledge of the fate that awaits Cybertron to rally both Decepticons and Autobots together and defeat the Chaos Bringer.
Ten years after the defeat of Unicron, Over-Run showed Prime how to commune with Cybertron itself. Doing so reformatted the Autobot leader into a body capable of Powerlinking with a team of 'drone' vehicles, and granted him (and the other Autobots) the ability to combine with each other.
Upon returning to Earth, he decided that the Autobots would no longer hide from the inhabitants, but work with them openly. This decision led, among other things, to the Autobots' induction into the UN.
Optimus Prime was dueling with Megatron atop a very high building just before Megatron received the Mini-Con beacon. Following his nemesis to prevent him from using the Mini-Cons for evil, Prime arrived on a new planet: Earth.
Arriving in the Amazon, he familiarized himself with the territory before going and recovering the first Mini-Con Panel. (Which Mini-Con it is varies with the game difficulty, with Lock-On being released on the first two difficulty levels). He then traversed a disproportionate bridge, had a brawl inside a temple, and scaled a pathway up a cliff face before arriving at a run-down temple and encountering Cyclonus. Cyclonus (sporting a fanciful, comic British accent) ordered a Heavy Unit to attack him, declaring Earth as "Decepticon property". Prime took care of the Heavy Unit, and Hot Shot and Red Alert joined him. Soon they discovered their old Mini-Con partners. (Sparkplug remains with Optimus for the remainder of the game, activating a rather pointless "Powerlinx" Max Payne-esque bullet time mode which drains Prime's health.)