Jeez. I love a Silver Age cover because they never beat around the bush -- you're generally shown exactly what the stakes are and given a pretty good idea of what the story's gonna be like.
Believe it or not, for the first time the first panel isn't a duplicate of the cover; I think I know why, and I'll cross that bridge when we get to it. Also, turns out "the Sun Brain", or maybe "Solar Brain", is basically exactly what it sounds like.
Maybe it's me, but didn't they just get back from space? And think about it, every time they went to space or another planet, something bad happened -- they brought back moon microbes, became stranded on a planet of giants, and Doc turned into an evil robot. I know it's a big place, but if I had a terrible experience every time I went to China, I'd probably stop going after a while.
This issue starts out quite a bit like #2 did -- the Metal Men and Doc are just kind of chilling. Platinum gets Doc some water when she thinks he's overworking himself, Lead shields Doc from the blazing sun at the beach, that sort of thing. The captions note that Doc might complain sometimes, but he really does appreciate how much his creations care about him. Doc also takes Platinum boating (on a boat constructed of Gold, Lead, and Iron, no less) in the park; Platinum can't help but feel this is a romantic gesture, and to be honest I'm kind of with her on this one, Doc is kidding himself if he thinks this is completely innocent.
A month later, however, Doc is about to shoot himself with the Metal Men in that gun form from the cover -- against his (and their) will! I think this is why the big first page panel wasn't the image from the cover, since it shows up just a few pages in, and again towards the end.
Yeah, this was all epilogue, or in media res or whatever. And we go from stuff we'll see again in a minute to stuff we saw before -- a flashback to the end of the Gas Gang debacle. As Doc recovers from the illness brought on by having his robot parts turned into steam, Platinum cares for him. Doc, naturally, is bellyaching the whole time and insulting her 'delusions of womanhood'. Later, Platinum bakes Doc a pie, but when she goes to give it to him sees he has a visitor - a model named Ava Woods, who wants Doc to get well soon so he can judge a beauty pageant. Ava makes a patronizing comment about Doc's unconvincing "suit of armor" robot girl, and as you can imagine gets that pie right in her face. Platinum vows to enter the pageant herself and beat Ava.
Elsewhere, Gold, Lead, Iron, and Mercury are continuing Doc's research projects while he's ill. If Doc told them to do this, I have to say, he really should have known better. They're tracking solar prominences, eruptions from the surface of the sun, when one of them breaks free of the star and starts careening through space. It gets blasted with cosmic dust, hit by a meteor storm, and caught in a nova. Because this is a comic book, that "cosmic energy" turns it into the Solar Brain!
Iron mistakes the creature for a childlike, curious being, especially when he sees it put the rocket back together. However, it places the rocket on a collision course with another spacecraft, destroying both! Soon, Doc has recovered enough to return to his work, and the Metal Men tell him about the Solar Brain. They've come to the conclusion that the Solar Brain can force objects to do its bidding - which is why the manned rocket flew straight into the other spaceship after it put it back together. Mercury thinks it can easily control living beings as well.
Later, Platinum enters that beauty pageant, and manages to do quite well -- the judges and other contestants think she just went wild with body paint for a laugh, and are stunned by her radiance. She's a shoo-in to win -- until Doc, who is judging the event after all, reveals she's a robot, disqualifying her. Ava takes home the crown or sash or whatever, and Platinum is so incensed she wraps Ava and Doc up in wire and spins them like a top. Ever the drama queen, she says she never wants to see Doc again and is going somewhere he'll never find her. Before he can do anything, Doc gets news that the Metal Men have found the Solar Brain again, and rushes home. However, a building collapses as he drives by, which Gold attributes to the Solar Brain. Tin (who was with Platinum the whole time) dies protecting Doc from falling girders.
Calling Iron over to carry Tin's remains home, Doc heads home and leads the Metal Men to his new experimental rocket. He hopes to repair Tin in the rocket's Metal Recovery Room, of course. As they blast off, Platinum runs after the rocket, regretting what she said. She manages to tie herself around the rocket before it's too late, and is reeled in by Gold as soon as Doc notices.
The rocket is nearly flattened by a huge meteor, but Doc activates his new "chain reaction" defense mechanism, wherein two mechanical arms extend from the rocket and grab whatever's approaching, which completes the reaction and blows up. That's when they pick up the Solar Brain again, and see it tearing apart a space station.
Reports from Earth and readings from the rocket indicate that the Solar Brain is, for whatever reason, hoarding weapons, any it can get its hands on. They track the Solar Brain to a small planet, where they find the weapons all over the place, as if they were toys the Brain was done playing with. Soon the Solar Brain ambushes them, tearing apart their rocket before putting it back together with the power of its mind -- and when the Metal Men attack, it decides to turn them into its toys. Also, it's not really mentioned, but Tin seems to be up and running again. And it thinks of weapons as toys, and since you saw the cover I think you know where this is going.
I imagine everyone saw that coming, I just included the pic because I thought it was really cool.
The Solar Brain's... plan? I don't really know how conscious it is. But its "plan" for is foiled when Lead, who is not just the chamber but the bullet himself, intentionally backfires rather than shoot Doc. However, this blows the Metal Men gun to pieces, incapacitating them all. Doc is forced to defeat the Solar Brain all by himself, and tries to accomplish this via the Chain Reaction
Obviously, that's the end of the Solar Brain, bless its demonic manchild heart. Doc repairs all the Metal Men in the Metal Recovery Room, but it's not mentioned whether the rocket's large pieces are fitted back together or if they're on the way back to Earth. Naturally, of the bunch, Platinum recovers first, and she and Doc basically do what they do: "Doc, I love you!" "Girl, you're a robot, shut up."