Warning: Spoilers ahead.
The series begins with narrated lore. About how beautiful stones rained down from the sky one night and landed on earth. They held mysterious power. Some time later, onyx stones landed, filled with terrifying creatures. The creatures were killing every living thing on earth, until Musicarts and Conductors came and saved them.
In this world, people no longer play music. A piano is an unknown object to a little kid. The main character, Takt appears. He starts playing an antique piano. He’s so amazing and it’s so rare, every takes notice of him.
The music draws an extremely powerful creature to Takt’s playing. It’s about to destroy Takt, but a girl who’s crazy strong comes in the nick of time to defend him. The girl’s dress morphs into a beautiful red one and she throws a baton to Takt, whose arm has dematerialized for some reason.
After an awesomely animated battle, the girl destroys the creature with a futuristic sword. We can safely assume Takt is a Conductor and the girl is a Musicart.
They’re traveling cross country to the New York Symphonica. The girl talks like a robot and her name is Cosette. Another girl comes by in a car. Her name is Anna and she’s Cosette’s older sister. She picks them up.
The three stop at a diner to learn the town is overrun by those creatures, D2’s. Cosette, after consuming calories, believe she’s strong enough to destroy all those D2’s. She does well until an unusual D2 beats the crap out of her. Since Cosette is connected to Takt and out of power, Takt also collapses.
They come up with a plan to draw out giant creature by Takt playing piano. It works. It’s a close battle, but Cosette is able to destroy the creature’s Black Night Siderite stone, and defeat it. The town is saved, and the three continue to drive to New York.
In Episode 2, we get a flashback. Takt’s father was a world famous maestro killed by a D2. So Grand Maestro Sagan directs all people to stop playing music, as it draws the D2’s. And Takt was traumatized and becomes a shut-in who does nothing but plays piano.
Cosette and Anna take care of Takt. Cosette was actually a normal human back then. She and Takt were always bickering, and Anna was always breaking up their fights.
She gets Takt to play in front of people at a festival. It’s a sweet moment, but the moment is ruined when D2’s rain down on the event.
The assault breaks Takt’s right arm. And seems to kill Cosette. But all of a sudden, a spirit girl comes out of Cosette’s stone. It instantly takes Takt’s arm, and that sacrifice is used to turn Cosette into a Musicart! Now she talks like a robot, asking for commands from Takt. Still a lot of questions about what’s going here exactly.