Darkness gives way to reveal the words: "Everyone is entitled to their own sorrow, for the heart has no metrics or forms of measure. And all of it... irreplaceable." The black closes in again just as an amplified voice is heard saying: "Ladies and gentlemen, please welcome...Weiss Schnee." A young girl in white with a barely visible scar over her left eye walks out on stage into the spotlight as a piano plays a song before a crowd of people, occasionally taking pictures from their stands around the white castle they appear to be in. She looks to her right and sees a silver haired girl in a silver dress and white gloves with the Schnee symbol on them, showing that she is family, as she gives an encouraging smile. Weiss smiles back, but it then disappears as she closes her eyes and begins to sing, "Mirror, tell me something. Tell me who's the loneliest of all..." While she sings, the view sinks down into her scar-free reflection on the stage, the world tilting until she opens her eyes and finds herself in another place, with the girl from before standing next to her. "Vanessa?", she asks the girl, but looks away as the light and shadows shift until a warrior in giant armor with an equally massive broadsword appears before them. They backflip just as the Giant Armor brings its blade down on where they were, and the song continues as Weiss' flashback takes over. Weiss continues to sing, "Mirror, tell me something. Tell me who's the loneliest of all?"
Weiss unsheathes her own rapier, Myrtenaster, while Vanessa releases her fan, Icy Vengeance, which has a grey color silk with a silver frame and the Schnee symbol spread on it, and dart forward, getting a hit on their enemy before twirling into a landing. The Giant Armor swings again, but they dashes away and land several spinning slashes on the creature. It retaliates when they uppercut into the air, arcing its sword around and hitting Weiss' rapier and Vanessa's fan, which they bring up just in time to merely knock them back. When they slow down, Vanessa looks up to see the Giant Armor leaping into the air and hurling the sword down on her and Weiss, then swinging it back up in an effort to get its fast-moving targets. It finally hits Weiss' blade again, and she is knocked away until she rolls back on her feet. Weiss still singing, "Fear of, what's inside of me. Tell me can a heart be turned to stone?"
With white energy now gathered on her fingertips, Weiss motions to the ground beneath her, and two snowflake symbols appear that propel her and Vanessa forward at extreme speeds. Despite the Giant Armor's attempts to slash at them, Weiss and Vanessa disappear and reappear all around it, leaping through the air using Weiss' glyphs and landing a blow with each pass. After a slash to the face, the Giant Armor aims at Weiss' feet, who leaps over it and gets an undefended fist to her head. Weiss is launched yet again, but this time, her attempt to get back up is weak and unsure. A worried Vanessa looks to Weiss, but then shakes her head and glares back at the enemy. A worried Weiss closes her eyes, and in the real world, the spotlight suddenly cuts out.
The moon makes itself known when the clouds in front of it pass, showering Weiss in its light as she sings, eyes still closed. In her mind, she stands back up, first looking at Vanessa as she points to the Giant Armor's broadsword and nods her head. Weiss then nods back and eyes the Giant Armor with blood trailing down the left side of her face, and holds out her sword, spinning the revolver in the hilt until a red energy appears over the blade. While this happens, Vanessa grabs the end of her fan and moves her arm outward, which causes the fan blades to harden and extend outward until the become the blade of a sword, with it's hilt appearing shorty after. The Giant Armor runs up and drops its sword on them, but it meets their rapier and sword and is deflected off. Weiss singing still, "Mirror, mirror, what's behind you? Save me from the things I see! I can keep it from the world. Why won't you let me hide from me?"
Weiss switches it up and spins, a cyan light now creating a wave of ice when she strikes it into the ground that freezes the Giant Armor in place. It swings again, but Weiss and Vanessa run ahead and jump on the sword. Myrtenaster is now glowing yellow, and they roll on the broadsword and slash at their foe, destroying the ice crystals in shards of purple but impaling its weapon into the ground. Weiss rotates the hilt again and a snowflake circle spins under her and Vanessa when the Giant Armor tries to hammer its fist down on them, but they roll out of the way and Weiss activates the glyph, launching the warrior up into the air and becoming trapped by Weiss' beams of blue energy. Weiss continuing to sing, "Mirror, mirror, tell me something. Who's the loneliest of all?"
With one final leap into the air and her real self singing the last lines of her song, Myrtenaster glows with the white markings on its blade and Weiss and Vanessa descend onto the Giant Armor. When their attack is done and they land in a crouch, the black hollows of the enemy light up and crash to the ground in a shower of ice and snow behind her. Weiss singing as the memory flashes between her last note and the battle, "I'm the loneliest of all." Weiss gets up and closes her eyes, finishing Mirror Mirror to a gentle shower of snowflakes and a roar of applause from her audience. She opens her eyes and looks around as if in a daze, then peers up at the slightly-scattered moon above. She then looks over to her sister as she applauds. She smiles at her, then dips her head and bows to the crowd as the curtains close around her.