There were technically five Sleepers in all, which would seem to indicate that the Skull had put into place one contingency plan after another after another, which under other circumstances would be almost laughable. Even in comics, it would have been improbable for Cap to come across a Sleeper on separate occasions, since you'd begin to think that building Sleepers was the only way the Red Skull spent his time during the war--which ties in with the notion that the Skull apparently thought it necessary to have other Sleepers built in case the original plan failed. Why not instead funnel those resources into your current problem: winning World War II? Why spend so much time and effort (and considerable secrecy) on projects that were contingent on the Germans losing the war?
The story of the Sleepers opens in 1966, as Cap shared one-half of the Tales Of Suspense title with Iron Man and a number of his stories recounted his wartime exploits. One of his memories of that period involves his final battle with the Red Skull during the closing days of the war, and a deadly warning that issues from his foe even as he may be breathing his last.
Heh heh--these may well have been the first *ahem* "sleeper agents."
As the Skull reveals, this first story features three Sleepers, with three different Nazis entrusted to activate them on the proper day. Our first thought might have been that they were geared to go into action separately--but as we take a look at the emergence of each one, we'll slowly learn that the Skull's engineers had something else in mind.
(Good grief--the thing is even goose-stepping.)
While Cap has acted on his fears too late, he's on hand to meet each threat as best he can; but as the second Sleeper makes its appearance, he realizes he's out of his depth and that he'll need to bring in the army on this one. Fortunately, he's headed toward the base of General Logan, whose sharp banter (courtesy of writer Stan Lee, who's clearly having a ball with the character) makes it clear he's all Army and nothing but Army.
From Cap's observations, it's clear that the Sleepers are linking up--but to what end? From their design, their threat appears to be rather unsophisticated--all that's missing is a close-up of the rivets holding them together. We should bear in mind that these constructs were designed in the 1940s, from available parts and materials--and since they're not doing a bad job of destroying towns and countryside, it's probably a good idea to take them seriously. Which becomes terribly hard to do when the third Sleeper is activated, and joins with the other two to make one god-awful looking piece of junk.
Very deadly junk, if Cap's theory of its purpose holds true. Having proven to be invulnerable to both ground and air attacks, and its course taking it north, Cap believes that the Sleepers are now combined into a massive bomb--and when it reaches the North Pole, its force rays will blast to the Earth's core and set off a thermal chain reaction which, combined with the integrated Sleeper's explosion, will destroy the planet.
In a desperate maneuver, Cap then boards the Sleeper armed with a flamethrower--which, yes, succeeds at damaging the Sleeper where the slightly more incendiary rockets and missiles of the U.S. Army failed. You can almost hear the chords of the Star Spangled Banner as Cap witnesses the scene.
The fourth Sleeper can thankfully be taken a little more seriously, as the Red Skull--who has survived his apparent death--unearths the Sleeper's crypt and prepares to activate it with a special sonic key.
This Sleeper, in essence, is a living volcano--able to move freely beneath the ground like a mole (thanks to its additional ability to turn intangible) until it reaches a target (usually a production facility or other installation), and then causing itself to build in heat and pressure until it "erupts," utterly destroying the target above it. Cap has captured its sonic key, after the Skull was unable to use it to assert his control over the Sleeper--but will he be able to stop the Sleeper before it causes untold destruction?
Why, yes, he will be able to stop it--because true love, in the form of Sharon Carter's frantic worries over Cap's safety, can resonate through the key and defeat even a Nazi doomsday device.
That's four Sleepers up to bat, and four defeated almost embarrassingly easy. You just know the Skull must be livid right about now. "Heads will roll when I see those engineers... oh, wait, I think I terminated them back in the '40s..." Unfortunately, the Skull had those engineers working overtime, because he has yet another of these puppies waiting for the signal to burst out of its burial site.
No, I don't know how the Skull managed to transport this thing to the States and implant it in the Nevada desert in the 1940s in secret. On the bright side, writer Gary Friedrich is insistently calling it the final Sleeper, so maybe it won't put us to sleep this time. And speaking of being put to sleep, you'll never guess what deadly power the Skull has given it which he believes will make it invincible:
...unless its attackers, oh, I don't know, wear gas masks to protect themselves long enough to get close to the Sleeper and destroy it. Or maybe simply attack it with long-range artillery. (Though shelling the first Sleeper(s) didn't accomplish much, did it?) Fortunately, the Skull brings about his own defeat, when he allows Cap to come inside the behemoth and get his hands--er, shield on its control panel.
Another embarrassing end for one of these constructs--and for the final time, if we're to believe Friedrich. Au contraire--read the fine print. We were told this was the final Sleeper--nobody said anything about pulling the plug on any further battles with Sleepers. What a disappointing technicality. We have Machinesmith to thank for reviving the fourth Sleeper, hoping to use it to infiltrate Avengers HQ. It seems that this Sleeper has lost its intangibility and volcanic abilities, which means that Cap can prevail against it even without Agent 13's emotional support.
Machinesmith is also on hand when the Red Skull, injured during the Acts of Vengeance, is taken by Crossbones back to Skull House, where one of its rooms contains functional replicas of devices the Skull has used in previous schemes--including, you guessed it, all five Sleepers. When Cap and his associate, Diamondback, investigate, Machinesmith acts to stop them with the tools at hand.
(Did these two even break a sweat?)
If memory serves, this would be the Sleepers' curtain call, at long last. As tenacious as the Skull seems to be about activating these monstrosities, there might be even more of them twitching beneath the ground and waiting for their signal--but in the name of mercy, let them rust in peace.
We've seen the Nazis give the Sleeper concept their best shot--
now let's see what happens when the Allies take a crack at it!