Things start off relativey normal by standards of anime and post-apocalyptic dystopian sci-fi. We meet our heroes Simon and Kamina in an underground city. Kamina dreams of breaking through to the surface, following his father and achieving greatness, a plan which requires Simon the digger and his trusty drill, who unfortunately for Kamina just wants to do his job and live an ordinary life. Then Simon stumbles across a drill shaped pendant and a large meetal head which appears to be activated as a fully functional robot by the drill trinket. At first, Simon just thinks it's a curious oddity, but he isn't allowed to just let it be for long because an even bigger robot drops in to the city and Simon and his robot, dubbed Lagann by Kamina, along with Kamina suddenly become responsible for saving the city. The fight ends with Team Gurren(also Kamina;s idea, most of the crap in this series is Kamina's idea) breaking through to the surface along with Yoko, also a founding member of Team Gurren who came from another city called Litner. Once they reach the surface they find that these robots are called gunmen and they are operated by beastmen who oppress humans and drive them back underground, only since Litner is destroyed, the people can't go back, so they have to fight, and now Lagann is their best hope of fighting against the gunmen. Then Kamina hijacks a gunman, something nobody else has either thought or at least has been brave enough to try, or lucky enough to succeed. Dubbing his gunman Gurren Kamina and Simon become a team to fight against the beastmen armies along with Yoko and her rifle and their mechanic Leeron. Gradually they crusade from village to village collecting allies and freeing people from the opprssion of the beastmen. Then comes the defining moment, when Gurren and Lagann face a foe that is too powerful for them to defeat, and Kamina comes up with the brilliant idea of combining the two gunmen by smashing Lagann on top of Gurren. It should be noted that the size ratio between Lagann and Gurren are the same proportionate to a human head and the rest of the body respectively, it should also be noted that beastmen have their arms and legs coming straight out of the head which doubles as the body, so what Kamina has done is give Gurren a head. One more thing that should be noted is that the way Kamina goes about this makes no sense whatsoever to anybody, not the viewer, not his allies, not the beastmen who actually know how gunmen operate, possibly not even to Kamina himself, but he does it anyway, and at then something happens that changes the direction of the show, when they completely abandon the laws of phyiscs and give in to sheer willpower, Kamina's plan actually works and Gurren Lagann actually ends up being a more powerful robot that can produce infinite drills to ensure victory for our heroes. From here, Team Gurren Lagann mangages to inspire an army of gunmen hijacking human warriors to stand against the beastmen. So far the story is still pretty straightforward with everyone going after the leader of the beastmen, Lordgenome, and culminating in the final battle.
Then things start to get wierd. Yes, wierder than they already were. Kamina dies duing the battle, but the survivors carry on and rebuild the world anew and in just seven years the barren wasteland of Earth becomes a fertile prosperous civilization. And then comes the anti-spiral race. As it turns out, Lordgenome was actually trying to protect Earth from the anti-spirals. There is a force called spiral power; why? Because the whole thing started with Simon drilling and the creator decided to run with that theme so spirals became the central theme of the series for no real reason. Anyway, the anti-spirals determined that spiral power would lead to the destruction of our universe, so to stop that from happening they decided to destroy any race that achieved threshold spiral power, defined as having a certain population level. By oppressing humanity and leaving varied beastmen at war on the surface, no spiral race was dominating Earth as far as the anti-spirals could tell, so for the greater good of the planet maintaining that status quo was actually in the best interests of Earth not being estroyed by the anti-spirals. Long ago, Lordgenome had attempted a braver, more straightforward course of action but failed and fell back on this plan. What has changed now is Nia, Lordgenome's daughter who is actually the artificially generated messenger of the anti-spirals. Due to the prosperity of the human race, they have achieved the threshold population and she has been activated. The problem is she was completely unaware of her true nature prior to activation and has fallen in love with Simon who will now literally go to the ends of the universe to save her. The final episodes go so far beyond the limits of what should make sense that the final battle is actually between two robots large enough to dwarf galaxies fighting in a pocket dimension universe. They stand on large galaxies and throw smaller galaxies. Read that last part as many times as you need to either make sense of it or for your head to explode. Seriously this whole thing went off the rails, but man did it look good doing it.
Honestly, I can't hate on this series for being ridiculous, pretty much every good anime goes off like this. They all start off making sense, then push the limits until it doesn't make sense anymore. Unless, of course, you have been watchign all along, if you follow the pace, the crazy sneaks up on you so you don't really notice it until after. While you're watching it, you're like, that is so awesome! And then after you're done watching you look back and you're all like, wait, what did they do again? Gurren Lagann takes honors in terms of scale because few have gone so far as pitting two entire planets agaisnt eachother on an intergalactic battlefield for the fate of the entire universe WITH THE ENTIRE UNIVERSE ACTUALLY VISIBLE DURING THE BATTLE! The series ended simply because there was no next step, they hit the end of the road, there was no getting bigger than it got, so cheers to Gurren Lagann for leaving it all on the floor-er, galaxy.