Anna enters the Loewengrin Marquessate as a maid to find a way back to her original world. This estate bears the ominous nickname, 'The Swan Grave.'

There, she unexpectedly encounters the Marquess, Rotbart. Mistaking Anna for his missing wife, he forcibly takes her and then proposes an unexpected arrangement:

“Become my wife’s substitute.”

He also reveals a crucial piece of information: for Anna to return to her original world, she must bear a child with someone from this world. While she deliberates, Rotbart’s son, Svanhild, also lingers by her side, pleading:

“I want a mother, Anna.”

A relationship with a predetermined end. Conditions that cannot be refused. Desperate to somehow return to her original world, Anna eventually agrees to Rotbart’s proposal...