In 1946, in Soror city, three different stories intersect amid macabre events. While detectives Thomas and Carlos work in the unsolved cases of the mysterious body found in a tree and Serena Caires disappearance, those three stories get together suggesting the motivations of those respective crimes.

In Slavic mythology, Rusalki are believed to be the spirits of young women who drowned in rivers or lakes. Their characteristics are similar to those of water nymphs, mermaids, and succubi. Rusalki tells the story of Leon, a historian who travels to remote cities to finish his thesis on historical places. Leon meets Liz, a widow who is struggling to cope with the recent disappearance of her only son. Liz allows Leon to spend a few nights at her house. However, unusual things begin to happen.