Originally just a security keycard, the Key possessed by Sari Sumdac holds a great power. Given a portion of the power of the AllSpark, the Key can be used to heal damaged Transformers, give life to machines, and control the machines. Such an object holds great power, and with great power comes great responsibility.
Sari had a little trouble getting a handle on that last part.
Oh, who are we kidding? She initially had no sense of responsibility when it came to the Key.
Fortunately, the Key itself has a better idea of when it should be used. Sometimes it can even overpower its owner to accomplish its own goals, even if they are contrary to Sari's wishes.
After Sari Sumdac met the Autobots, she convinced Bumblebee to take her to their ship. Once there, her inability to stay quiet forced the yellow and black bot to drop her down a shaft. She was deposited into the cargo hold where the AllSpark was located. Scanning Sari's mind, it channeled some of its power into the key. Optimus Prime discovered Sari, but was forced to rush to the med bay, where Prowl was going off-line. The Key began to glow, and she used it to heal his damaged circuits, saving his life. Several days later, Sari used the Key to restore Optimus Prime's spark after he was killed in the battle with Starscream.
While trying to get into her father's lab, Sari discovered that she could use the Key to control other machines. She activated the lock on the door, entering the lab, though Sumdac quickly moved her out of the room. The leftover energy, however, transferred itself to the head of Megatron, restoring his higher functions. Sari then used her Key to short-circuit her Tutor Bot and get out of class early. (Don't pretend you wouldn't have.)
When Bumblebee and Ratchet were creating an electromagnetic pulse to counter the Dinobots, Sari used the Key to give Bumblebee's stingers a boost. The AllSpark energy was channeled into the Dinobots, granting them sparks.
When Blackarachnia arrived on Earth, she tried to use the Key to purge her techno-organic body and restore her original form. However, the Key began to drain the life from everything organic in Detroit, and it nearly killed Blackarachnia as well. Prime was forced to remove the Key to save everyone. Blackarachnia was noticeably displeased with this.
Megatron had been observing Sari for some time, including her liberal use of the Key. He built Soundwave as Isaac Sumdac's birthday gift for Sari. In fact, Megatron had designed Soundwave to evolve with each exposure to the Key, with the intention of using it as his new body. However, Megatron did not foresee that the Key would also grant Soundwave sentience.
Sari used the Key to cheat at a game of street hockey with the Autobots. Later, the Key seemed to act as an early-warning system, projecting a beam of light whenever trouble for the Autobots appeared. When Blitzwing and Lugnut appeared on Earth, it projected a beam of light targeting them. Later on, it projected a similar beam of light at a series of missiles launched at the Autobots. Once the Autobots eluded those and Ratchet safely got Sari to their ship, the Key prevented her from sabotaging the vessel so her friends couldn't leave, and aided in repairing the weapon systems. It also not-so-subtly gave Sari a little jolt to remind her of the power she wields through it, working with the AllSpark to project holograms of Megatron. This scared her into a reconciliation with Ratchet and indirectly led her to realize how she'd been abusing the Key. This time, it looks like the lesson may stick.
Unfortunately, Optimus Prime didn't learn the right lesson. Although Sari had been chosen as the guardian of the Key, Prime was convinced by increasing Decepticon activity that it was no longer safe in Sari's care. He ordered Ratchet to confiscate it from her...and shortly thereafter, Ratchet was ambushed by Blitzwing and Lugnut, who stole the Key. They delivered it to Megatron, who used its power to rebuild himself, just as the foreboding vision had warned.
In Megatron's hands, the Key proved capable of harming machines as well as healing, used by the Decepticon leader to offline his traitorous second in command Starscream. Megatron was also able to use the Key as a beacon to the AllSpark itself. However upon acquiring the AllSpark, Dr. Sumdac stunned Megatron and dropped the Key. Sari seized this opportunity and tossed the tiny object to Optimus Prime, who used it to shatter the shell that kept the AllSpark's energies in check.
In the wake of the shattering of the AllSpark, the Key proved effective in locating the scattered shards of the artifact. Unfortunately it also showed such endeavors to be a bit hazardous, as it could not be removed from a piece of equipment that had a shard lodged in its workings. Optimus Prime's axe was needed to destroy the machinery and free the Key.
Later, after finding out she's part Cybertronian, Sari used the key to upgrade herself. This action turned out to be disastrous because she went out of control and the key, having all the power drained from it, was now useless.
- The Key resembles the Key to Vector Sigma from Generation One. Both artifacts grant access to the life giving power of their respective hosts, and both can have potentially disastrous results if misused.