The title says it all. I didn't notice this until the second or third time I watched One Shall Rise, Part III. But first, let's review. Optimus gives Jack the Key to Vector Sigma, and Ratchet assumes it's because Optimus doesn't believe he will survive his confrontation with Unicron. And you can see it in how Optimus acts. He doesn't face the humans or anyone when asked what will happen to him. And he choses Jack to hold onto the Key because he's impressed by how much Jack has grown since he met him. Ratchet also mentioned that Optimus was once like Jack in his younger days. What do the Key and the Matrix have in common? They are both dead-vital and important artifacts to Cybertron. Which means, they are both MacGuffins. What do MacGuffins do? And what did the Matrix do in Part III? "Light our darkest hour". Jack now holds basically another Matrix to "light our darkest hour." Things look pretty bleak right now, don't they? The Autobots are leaderless (until either Ratchet or Arcee step in as acting leader). Megatron has Optimus wrapped around his finger as his protege. And Optimus is, well, Optimus. Watch how badly the Autobots get their asses kicked if Megatron siccs Optimus on them. This is another "darkest hour", and things are no doubt gonna get worse before they get better. So Jack has the Key. His Matrix. With which he will save the day and light the darkest hour by bringing Optimus back to being Optimus. "The Key to Vector Sigma will light our darkest hour." The Key is what is going to bring back Optimus. Now let's look at the Matrix again. We see this shot of the Matrix right before he uses it to kill Unicron and he loses his memories. Why would we see a shot of the Matrix within Optimus when we're about to see it in its' full glory saving the world? It's blink or you miss it foreshadowing. Hello ladies. Look at the shape of the "heart" of the Matrix. Now look at the picture of the Key. Look at the shape. Matrix, Key. Matrix, Key. Matrix, Key. The Matrix and the Key are about the same shape. Optimus gave the Key to Jack because he believed him to be the next "Prime" who would light their darkest hour. Optimus likely knew the risk of death was very high as it is, but in case he did lose his memories, he gave it to Jack (for all we know, he knew losing his memories or death were his options, hence why he didn't face the humans when asked). The shot of the Matrix before Optimus uses it is the hint to us to realize, that's how he will come back. Jack will take the Key, unite it with Optimus's Matrix, and it will restore not only the wisdom of the Matrix, but Optimus' memories himself, and he will become Optimus Prime once again. The shape, while not perfect, could be fixed. Yes the Key isn't the exact same shape, but we saw it transform into a smaller form for Jack. Who's to say it can't transform to fit the Matrix? It's likely where Optimus has kept the Key all this time anyway. But that is the biggest hole in my theory right now, the shapes not being the same. But even then, maybe it doesn't need to be a perfect puzzle piece, but it could simply be jabbed into him like Sam did with the Matrix in ROTF. So, any thoughts? Comments? It's not a perfect theory, but it's an epiphany I had.