Promethus Unbound is the sixteenth episode of the Beast Machines TV series.
A giant Cybertronian moon passes (rumbles, actually) overhead. Noble howls at it, much to the consternation of the distant Maximals—their new friend has run off yet again. Nightscream blames the team's treatment of the newcomer. Cheetor gently defends their actions as being for Noble's own good. "And ours," Rattrap adds.
Blackarachnia is vaguely unsettled by Noble; Rattrap is freaked out. Nightscream doesn't take this well, and he storms off with Optimus in pursuit. Rattrap and Blackarachnia note unhappily that Noble is probably heading for "the same place as the last five nights".
Noble is furiously banging on the doors to Megatron's citadel when Nightscream and Optimus arrive. He transforms to Savage form when they confront him, then continues trying to claw his way in. A bunch of snaking cables worm out to attack Nightscream when he tries again to stop Savage—and Savage knocks him out of their way. The cables envelop and constrain him, and the Maximals peel him free to find him back in Noble form.
Noble seems confused about how he got there and says the Maximals should lock him away. But Optimus defends him, saying he saved Nightscream even in his Savage guise. Rattrap isn't buying it. Nightscream nastily puts Rattrap back in his place, saying he knows what it's like to be alone.
As usual, Optimus calms things down, focusing the team on finding the missing Sparks. The group prepares to infiltrate the citadel to look for clues. Noble wants to help, saying he's been drawn here for a reason, but Optimus refuses—they're going to have enough to worry about. Nightscream supports Noble, but Optimus doesn't budge, and Nightscream storms off.
Rattrap plugs in and the doors slide open. He's then restrained from rushing in by Blackarachnia, as Optimus reveals a destructive force field across the entryway. A quick shot to the control panel takes it out. Too easy—this was the trap they were meant to find. The group enters...and Noble, some distance away, turns and sprints through the doorway. A pursuing Nightscream follows just as it closes.
Optimus is ticked off, but Noble says he couldn't help himself and seems to be having trouble controlling himself.
As Rattrap begins a search for the security shut-off, Noble takes off running for the control room...and promptly steps on a floor sensor, activating a field of floating mines. The Maximals freeze, but just then Noble's eyes glow red and he morphs back into Savage, setting off a string of the mines. KaBOOM!
The group groaningly gets to its feet, and Rattrap chews out Nightscream for dragging Noble along. They hear distant roaring, and split up—Nightscream and Optimus hunt for Savage, while Rattrap heads for the throne room with Cheetor and Blackarachnia as backup. As the two groups proceed, both Savage and Noble watch from the shadows.
Nightscream hears Savage's roaring and takes off after him. Meanwhile, Savage changes back to Noble and opens the doors to Megatron's throne room. "Success!" he says blissfully, as he sees Megatron's empty control harness. Noble begins leaping across the floating jigsaw puzzle bridge, but loses his grip on one section and falls. Nightscream and Optimus arrive and go after him.
Rattrap and company arrive a moment later and stare at the drifting pieces of floating bridge. Cheetor and Blackarachnia go first and offer the less-mobile Rattrap assistance in getting to the harness. He plugs in, and the bridge assembles itself under their feet. Screens of data assemble before Rattrap. He begins searching, but everything's encrypted. Optimus encourages haste as he arrives with Nightscream and Noble—and the latter's gentle meekness abruptly disappears.
Noble exults in being in the throne room, then spots Rattrap in the harness and immediately morphs to Savage. The Maximals realize he can control his changes. Savage blasts Rattrap out of the harness. Immediately, the bridge deconstructs and alarms go off. Tentacled Spark extractors slither out of the walls. The group (including Savage) makes quick work of them, and Rattrap gets back into the harness as Savage morphs back to Noble, standing next to the harness. Rattrap starts to show gratitude for Noble being back in control—but Noble yanks him out by the tail and, in a dramatically backlit scene, tosses him away. He enters the harness himself, but nothing happens.
Noble realizes he can't interface while in an organic body. He leaps clear, grabs two Spark extractors, and applies them to...himself. A furiously burning orange Spark emerges, and the body collapses. The Spark enters the control harness and shoots up the wires as Megatron's voice cackles: "Free...at last. Free at last!"
The Maximals charge outside to see the giant Megatron head now fully animated. Megatron delightedly explains that "Noble" was just a ruse, his gigantic eyes glowing in synch with his words. Nightscream is crushed. Megatron explains further: "For an eternity, I slaved to free myself from the organic material contaminating my otherwise perfect technological body. I finally succeeded in separating the two, but my Spark remained with the wrong half—the organic half. I knew that the answer to curing myself lay inside my citadel, but as an organic beast, I could never hope to enter it and survive. Which is why I needed you...Nightscream. Naive. Headstrong. And utterly malleable. You proved the perfect pawn...and now, in gratitude, I will free you from your organic bodies."
The big floating head fires a beam into the planet, and all systems come back on-line—including the drone army, which storms to the attack. As the others fight them off, Rattrap opens up a panel, and Optimus orders a retreat. Nobody objects as the badly outnumbered group scurries underground.
The Maximals lament their faux victory. Nightscream is still shattered, and Rattrap offers his sincere sympathy.
Back in the empty citadel, Noble's eye opens, and he stirs with a growl...