The year is Post Disaster 323. More than 300 years after a great war between Earth and Mars known as the "Calamity War", Mars has been successfully terraformed and colonized by humans. However even with the technological advancements, the humans on Mars crave freedom against the government of Earth and seek to improve their livelihoods. Furthermore, while most of Mars' nations have received autonomy, the planet is virtually dependent on Earth for economic development. A young orphan named Mikazuki Augus is part of a group of children who work for the civilian security company Chryse Guard Security, entrusted with the safety of the Martian Princess Kudelia Aina Bernstein, who has plans for her nation of Chryse to secure independence from Earth. One day, the group was attacked by the Earth Military organization Gjallarhorn, which aims to stop the Martian independence movement. During the attack, the leader of the children and the Third Army Division within CGS, Orga Itsuka, decides to rebel against the adults and higher-ups who had escaped and left the children and foot-soldiers to remain and fight as decoys. Augus must now choose to either follow his friends, or his mission as he stumbles into the legendary mobile suit: the Gundam Barbatos.

Rock n' Roll may never die, but the Sakuragoaka Girl's High School Light Music Club might not be here to stay unless would-be drummer Ritsu Tainaka can find three more members to replace the graduated seniors. Determined to see that the beat goes on, Ritsu can get a little help from her friend Mio Akiyama on bass, and together they might be able to convince choir-inclined Tsumugi Kotobuki to join on the keyboards. But even then, they'll still need a hero, preferably of the juke-box variety with stars in her eyes. Instead they find Yui Hirasawa, who's looking for a club to join and who's never held a guitar in her life. But when Yui does pick up a beat-up six-string, it feels good in her hands, and it doesn't take long to understand that maybe she's finally found a place where she belongs. Because you can't stop the music when four girls share a dream, even if it's something as crazy as someday playing at the famous Budokan. It's going to be a long and winding road, but if they find their passion and follow their muse, they could go anywhere in K-ON!

I-Machines are the general term for robots that operate in extreme environments. While Alliance Academy student Maya Mikuri is in the middle of operating an I-Machine, she gets involved in an incident with pirates, and ends up serving as a crew member on an excavation company's spaceship.