Action Masters are Transformers that traded in their ability to transform to become harder, better, faster, stronger and more alive. As the Autobot/Decepticon war drained more and more of their resources, a new source of fueling, Nucleon, was discovered which would grant Transformers amazing power, but at a cost. To compensate for their lost ability to transform, the Action Masters built transforming partners and (massive) vehicles to aid them in battle.
A few Action Masters managed to retain (or possibly regain) the ability to transform; they are known as Action Master Elites.
Toy packaging blurb
According to the toy packaging Optimus Prime went searching for new energy sources and, within the heart of a black hole discovered Nucleon: a miracle fuel that made Transformers stronger, faster, and more alive, but cost them the ability to transform. Soon the Decepticons stole the technology and created Action Masters of their own.
This story was later referenced with the Japanese release of Nucleon Quest Convoy, a black repaint of Powermaster Optimus Prime who was given the backstory of being Prime specially outfitted for this trip into the black hole.
Marvel Comics continuity
Grimlock heard rumors of a new miracle fuel on the planet Hydrus Four. Against orders he stole the non-functioning bodies of his fellow Dinobots and headed there to find out. What he discovered was a ruined world filled with creepy, obsessive, rotting zombie-like beings warning him that it was all because of Nucleon. Being Grimlock, he then went ahead and used the fuel anyway, first on himself and then to revive his fallen teammates.
Grimlock soon found parts of his body inexplicably seizing up, and began to fear that he should have listened to the creepy dying robots after all. After returning to the Ark Grimlock used the Nucleon to repair all the damaged Autobots in the ship's medical bay (as well as Megatron, whom he didn't know was there). Following the Transformers' battle with Unicron Grimlock froze up entirely. It was then that Hi-Q (in the process of merging with Optimus Prime) saw with "Powermaster eyes" that Grimlock wasn't malfunctioning, but changing. He used his Powermaster abilities to speed up the process and Grimlock burst forth in his new, more-powerful Action Master form.
Meanwhile, Hi-Q's own mental merger with Prime was also accelerated by his contact with Nucleon. Soon afterward, the Last Autobot re-created Hi-Q as Optimus Prime, looking very much like his Action Master toy and never being seen to transform. However, while Grimlock explicitly said that his own transformational abilities had been lost, there was no such statement for Prime. So while the involvement of Nucleon and Prime's general appearance suggest that he had become an Action Master too, the uniqueness of his re-creation makes things less certain.
- (Note: Other Action Master toys were represented in the comics, but not having been changed by Nucleon, they are likely not actual Action Masters. Simon Furman has commented that if the comic book had continued, we would have seen other Transformers become Action Masters, and we also would have seen a follow-up to the hints that Nucleon affects everyone differently, with some Transformers being affected by it in various alternate ways.)
As time progressed more and more Autobots, including Prime, Sludge, Snarl and Swoop, also succumbed to mode-lock. Prowl and Slag were among the few that could still transform, but they had doubts that this would remain much longer. Hearing rumours of further developments on Hydrus Four, Grimlock led the Dinobots there to see if a cure could be found. Amidst a Decepticon ambush, he discovered a new version of Nucleon that lacked the side effects of the original and restored the ability to transform.
- (Note: The events of Another Time and Place are hard to reconcile with the Generation 2 comic. However, in the latter, Grimlock's ability to transform has also been restored.)
Timelines Classics comic
As in other fiction, neither Grimlock nor Prime kept their Action Master bodies for long. However, Snarl is seen as an Action Master (and his partner Tyrannitron even gets a mention). Given Snarl's hatred of his own dinosaur mode, his remaining an Action Master was likely a matter of taste.
Optimus and Megatron's pre-G2 bodies share a striking similarity to their Action Master toys. Likewise Optimus Prime has been documented as searching out a New Energy source called Nucleon that's located within a black hole. Finally, Optimus Prime's Action Master was sold with New Years Convoy, and described as the intermediate form between Prime's various G2 Bodies.
The Action Master toyline, depicting the obvious oxymoron of "Transformers who don't transform," generated initial ire from some fans. This was magnified as Action Masters were released in the final year of the original Transformers toy line in the US. When Transformers was summarily canceled in North America, some fans blamed the gimmick for the death of the franchise (despite the wide variety of other factors that doubtlessly were involved).
One aspect Action Masters is notable for, however, is the mass reintroduction of "classic" characters. New toys for old characters was not a theme the original toy line visited very often, and so to have a single sub-line with so many recurring favorites was, at the time, a novelty. In addition, the classic characters were designed to resemble their cartoon counterparts more closely. For much of the Season 1 and 2 cast, the original figures having been designed years before they were stylized and simplified for the cartoon, these Action Masters were the closest likenesses of their fiction representations ever.
As with many other changes to the Transformers brand, initial hostility towards Action Masters from the fandom eventually became less intense and diversified into a broad range of opinion. Many of the most in-demand vintage (and therefore expensive) Transformers toys are Action Masters, and new characters from this subgroup are still hearkened back to in newer toy lines. For example, Axer shows up in the Robots in Disguise toyline, Armada Overload has design elements based on Rollout, and Cybertron Smokescreen's colors are a partial homage to Rad.
- According to the letter page of Marvel UK issue #300, Nucleon causes a Cybertronian's Figure Alteration System to convert to an Energy Storage Reactor, which is why Action Masters cannot transform.