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Megatron enters the Insecticons' lair on board the Nemesis and asks them who the strongest one is. A voice responds that while there is no more powerful force than an Insecticon hive, if he only has to choose one... and an Insecticon lands and transforms in front of Megatron, calling himself Hardshell. Megatron says Hardshell will retrieve something for him.
A Ground Bridge opens and Hardshell emerges with three Insecticons. They fly off in search of the fourth relic.
Back at the Autobot base, Optimus tells Miko and Jack that they'll be going with Arcee and Bumblebee on their mission. Bulkhead compliments Miko on helping get the new coordinates for the relics, and she thanks him before heading off, promising to tell him about her trip when she returns. Ratchet tells Bulkhead he's heading for the equator. Optimus tells him Raf and Fowler will be in charge of communications and transportation, but Fowler's still out of it from being electrocuted.
Bulkhead bridges out and arrives in the middle of some mountains. After being disgusted by the smell, he sets off on foot. He contacts base, saying he can't find anything. Raf reconfirms the coordinates, and Bulkhead insists there's nothing there... but then he hears the approaching Insecticons. Hardshell attacks, picking him up and carrying him for some distance while flying, then drops him. The Insecticon transforms and somehow knows that Bulkhead was a Wrecker, saying "I know your type." They fight, and Hardshell sinks his claws into the Autobot. Bulkhead rallies, blasts him, then uppercuts him, knocking one of his horns off his head. He transforms and drives off.
Back at base, Rafael tells Ratchet he's preparing to upload the virus. Bulkhead radios in and a recovered Fowler answers. Fowler says he spent his "beauty sleep" dreaming he finally got some respect from Bulkhead. Bulkhead thinks a long-ago lava flow may have sent the relic downhill from the provided coordinates. While reassuring Fowler that the relic surely survived some molten lava, Bulkhead realizes he's leaking energon from the fight.
Hardshell comes to as the other Insecticons find him. He notes that Bulkhead left him alive and says that will be his last mistake. The Insecticons fly off.
Bulkhead drives past a canister stuck in the ground and skids to a stop. After a brief struggle, he opens it to find that it is the fourth relic. He looks into the container and his expression turns grim. Contacting Fowler, Bulkhead tells him he's found a cache of energon -- but not the good kind. What he's found is called Tox-En, which causes dizziness and nausea upon contact, and weakens Cybertronians the longer they're exposed to it until it extinguishes their sparks.
Fowler tells Bulkhead to get out of there, thinking that the Decepticons won't want anything to do with it either. Bulkhead says no go -- back on Cybertron, Megatron found a way to turn Tox-En into a weapon of mass destruction, and he can't afford to risk this sample falling into Decepticon hands. Fowler suggests bringing it back to base, which Bulkhead also shoots down, saying that even if they could store it safely, it's too potent to risk exposure. He hears the Insecticons approaching in the distance and hangs up, then pulls out a grenade, saying he'll take care of it "Wrecker style."
The Insecticons land at the canister. One of them approaches and opens it, but as it's about to grab the Tox-En, the grenade explodes, killing it, and destroying the canister and the Tox-En inside. The dead Insecticon lands in front of Hardshell, who realizes what was inside it.
Bulkhead runs up a mountainside, carrying the rest of the Tox-En. Fowler asks him if he's nuts, and Bulkhead replies that he had to put some distance between himself and the Insecticons so he can dispose of the rest of it -- by dumping it into the volcano. He coughs up some energon after saying this.
Hardshell contacts Megatron to tell him that he's found the Tox-En... but he doesn't actually have any, saying that an Autobot has it. "Are you not the fiercest Insecticon?" Megatron asks.
Back at base, Fowler notices that Bulkhead's running away from the volcano, and still leaking energon.
The three remaining Insecticons find his trail. Hardshell tastes the energon, notes that it's fresh, then they return to the air.
Bulkhead, now in vehicle mode, is struggling to make his way back up the mountainside. Fowler compliments him on thinking to leave a false trail, saying that he used the same trick once when he got separated from his unit behind enemy lines. Bulkhead is curious, and Fowler tells him about his Army Ranger past, saying "we could wreck with the best of 'em." Optimus, on his mission, interrupts and tells Fowler that he needs to get into the research facility in the Antarctic.
Bulkhead continues to struggle to get up the mountainside, and he finally transforms and slips down. Gathering himself, he resumes the climb on foot.
Fowler gets the Antarctic base evacuated, the contacts Bulkhead, who asks if he's heard from Miko yet. Fowler tells him to focus on his mission, spurring him on like he would a soldier. Bulkhead slips again, falls and loses consciousness...
...but only temporarily, as he eventually responds to Fowler's communication. Exhausted, Bulkhead says he can't finish the mission. Fowler doesn't want to hear it and says he's been through worse. Several flashbacks through the series show Bulkhead dealing with "worse." Bulkhead says even Scraplets weren't as bad as this and wonders why he decided to do it this way. Fowler reminds him why he took the mission... honor, duty, family. Bulkhead flashes back again, this time to moments with Miko. Re-energized, he picks himself up and carries on.
The Insecticons have been following Bulkhead's false trails to a dead end. Hardshell turns toward the volcano and says they'll have to anticipate where he's going.
Bulkhead reaches the volcano's rim and tells Fowler he's almost there. Fowler encourages him to press on when Miko calls him.
As Bulkhead crosses the volcano, the Insecticons arrive and surround him. He takes one out with a grenade, then shoves the Tox-En in Hardshell's face as he attacks. He knocks the last Insecticon out of the air with a uppercut, then shoves the Tox-En into his body cavity while he squares up to Hardshell.
Meanwhile, Fowler converses with Vogel before hanging up, then reconnects with Bulkhead to tell him Miko's safe and on her way back. Encouraged, Bulkhead starts to trade blows with the Insecticon. Hardshell knocks him down and his vision goes blurry.
Ratchet returns from his mission to see the Iacon database downloading into the base computer as Fowler is desperately trying to raise Bulkhead, who's still fighting Hardshell. Fowler yells at the celebrating Ratchet and Raf.
Hardshell moves to finish off Bulkhead, but the Autobot moves and tosses him aside, right next to the Tox-En. Bulkhead knocks Hardshell back and throws the Tox-En at Him, causing him to stagger backwards into the volcano. He sees the rock dissipate and radios Fowler, who tells Rafael to open the Ground Bridge. As he's walking through, Hardshell climbs out of the volcano and fires at Bulkhead.
The episode ends the same way the last one did -- with Bulkhead flying through the Ground Bridge into the base, smoking and unconscious.
- Bookend Cliff Hanger: "Triage" and "Toxicity" both end with a gravely injured Bulkhead flying through the Ground Bridge and landing unconscious on the floor of the Autobot base. But where "Triage"'s ending asks "What happened to Bulkhead?", "Toxicity"'s ending asks, "What's going to happen to Bulkhead?"
- Call Back: Several to the last three episodes, which are all going on concurrently.
- Determinator: Bulkhead's always been this, but he takes it to the extreme in this episode.
- Fight Off the Kryptonite: Most of the episode.
- Flash Back: Three of them.
- No MacGuffin, No Winner: Bulkhead knows destroying the Tox-En is the only solution.
- Not So Different: This episode has Bulkhead bonding with Fowler over their military experiences.