The biggest mistake you can possibly make is to see mechs and assume that Rinne no Lagrange is to think of it in terms of Gundam or Robotceh. This mech series has some genre definition issues: though it dresses like a mecha anime, it acts like a magical girl series.
Rinne no Lagrange is set in Kamogawa, Japan, a fact the Kamogawa tourism department is pretty thrilled by. The protagonist, Madoka (not that Madoka) starts by saving the life of a drowning child and having her school uniform stolen in the process. The uniform thief is Fin E Ld Si Laffinty, an alien with a rather unique greeting and a strange fixation of Madoka. Madoka declares them comrades, and so Laffinty (just call her Lan) says that it now is time for Madoka to pilot a giant robot. Because that’s what comrades do, right?
From there, Lan gets her own robot, and a third figure Muginami sort of suddenly appears and gets a robot as well. The story has a lot of light and fluffy scenes of the aliens Muginami, Lan, and the three male mech pilots Array, Izo, and the blue-haired one who’s name is said maybe twice all adjusting to life on Earth, the big bad from the first half the series even plays with a strange hybrid of a Wii and a Kinect at one point. But so many of the lighter stories tie into the big, downright apocalyptic plotline.
See, Lan, Madoka and Muginami have special mechs, called Voxes, which may be responsible for almost wiping out all life on Earth in the world’s past. You know, no big. And the Voxes are also emphatically liked with their pilots. And sentient. So they kinda rock. Madoka’s Vox, nicknamed ’Midori’, even texts her at one point. But the Voxes have great destructive potential. For half the series, Earth is surrounded by planetary rings… of flowers… all because the Voxes can just do things like that. Y’know, for funsies. And not for nothing, but science has validated Lagrange’s depiction of Earth’s rings in the sky.
On the girl/girl front, we have an ever-rare girl/girl/girl setup! These three, Madoka, Lan and Muginami, made me believe in OT3. Lan is remarkably certainly in love with Madoka, being often jealous of Madoka paying any attention at all to Muginami. The writers tease the yuri content throughout the series, with lines said to the flower trio like “Who needs men when you three have each other?” The series does essentially canonized the OT3 with Madoka lamenting Lan and Muginami being off-world on Valentine’s Day, and thus the brunette missed her opportunity to confess to her alien ‘friends’.
Rinne is 24 episodes of fun, apocalyptic horror, and just enough loose ends left untied to keep writing it if they want to. The elements of this series you’ll enjoy, be they the sci-fi or the slice-of-life, will so massively outweigh the parts of the series that aren’t really your thing so as to make this series indefensible