Takahashi Rumiko Gekijou Ningyo no Mori

Synopsis

Mana is accidentally killed by a truck when she runs onto the road. Old Doctor Shiina tells the police and Yuta that she survived and wandered away, however he takes her corpse to the house of an old woman named Sawa who lives with a young woman called Towa. Shiina prepares to remove Mana's forearm to replace the deformed arm of Towa, but as he begins, Mana revives. Yuta realizes that Shiina lied and tracks him to Sawa's house in Mermaid's Forest, where local legend says a mermaid is buried. Towa commands a Deformed dog to attack and kill Yuta. When Yuta revives, he finds himself chained in a cell in the basement. Towa kills Mana, planning to have Shiina transplant her head onto Mana's body. Meanwhile, Sawa frees Yuta and reveals that she and Towa are identical twins, but when they were young, Sawa gave Towa medicine made of mermaid blood to save her from a terminal illness. The medicine made her ageless, but turned her hand into a Deformed claw and her hair white. Rather than admit his daughter was partially Deformed, their father had told everyone that Towa had died and confined her to the cell in the basement until his death.

(Source: Wikipedia)