What happens here is a NON-STOP action from begining to end. It is about human conflits over evolution and exagerated-consumption of resources, depleting everything on the name of science and growth, but at the same time, it focus on oppressed people, rationalization of resources that are scarce and dreams that people have over better days, about the forbidden door people are afraid of passing through.
It takes courage tough, and in a blink of an eye, the truth is revealed, things starts to move in an incredible speed, and with passion and determination (and a bit of luck?) there're victories and gains, thus providing the growth of the cast and the improvements/evolution of their past way of life.
In the majority of episodes people are at war, in conflict and fighting in a straighfoward manner, i mean, no plot twists and you'll get exactly what you expected.
The only thing in my vision that is inconsistent is Yoko. Fanservice Anyone ? C'mon the girl is supposed to have 14 years old and she has large breasts which girls that age don't have any...especially if they are slim like Yoko is. The only thing that is acceptable about her is that she gets enloved by someone of the cast close to her (NO SPOILS). "It is because a girl in that age would fall in love with a trashcan if it looked at her the right way." (Citation: By Baka-Raptor).
Finishing it, Tengen Toppa Gurren Lagann is a great action anime with very little romance, lot's of mechas and mechas destructions, with the political ideologies i stated before (grow and consume/opress and rationalize) being exposed and the "spiral" power everyone have in their own hearts that can be unleashed to make impossible things possible.
