Maria Gibson (Celia Gregory) tells one her servants that she is going to destroy the governess Grace Dunbar (Catherine Russel), because she's stolen the affection of her children Billy and Esther (Dean Magri and Daniela Roy) and her husband J. Neil Gibson (Daniel Bassey) from her.
The two women meet in the Thor Bridge, a place half a mile away from the Gibsons' summe manor house. Neil Gibson was an extremely rich American gold merchant and an ex-congressmen. Holmes takes an interest in the case.
Maria was found with a bullet in her head. They found the gun hidden in a drawer in a chest at Graces room. Maria was a Brazilian middle-aged woman, and Grace was much younger. However, the overseer Marlow Bates (Niven Boyd) says that Neil used to brutalize everybody, especially Maria. Maria was passional and a bit wild, compared to British standards, and suffered a lot because of Neil's behavior, who was constantly making her jealous. Bates wants to say that to Holmes before Neil Gibson arrives.
Neil arrives as a proud millionaire. Neil offers Holmes a huge sum of money, but he refuses it. Neil says that Grace is innocent and wants to save her. In spite of it all, Neil says that he didnt have any kind of personal relationship with Grace apart from her being his children's governess. Neil is about to hit Holmes, and threatens him. Neil leaves, but Holmes is sure that hell return. This doesnt seem to happen. Watson suggest to visit Grace in gaol. Watson gets all the paperwork right, so they go there, but Neil expels them from there. Holmes talks to Ferguson (Stephen MacDonald), one of the people who are with Neil Gibson. With Sergeant Coventry (Andrew Wilde), the local police officer, takes them to Thor Bridge. Holmes says that Maria didnt need to take the note with her. Coventry tells them that the ex-congressmen had many weapons at home. As Gibson doesnt want to talk to Holmes and Watson before time, they talk to Bates. Bates is not afraid of Gibson anymore.
Gibson finally accepts that he was in love with Grace. She had wanted to leave the position, but as she needed the money, she decided to stay if he doesn't bother her again. She had tried to convert him into a good man who wouldn't exploit his workers. Holmes and Watson visit Grace Dunbar in jail again. She had convinced Gibson of treating well some tribemen. At the Thor Bridge, in a very secret meeting, Maria shouts to Grace so Grace runs away, letting Maria threatening her aloud in Brazilian and English.
Holmes proves that Maria committed suicide, but posed it as a murder. She stole two of her husband's guns, and put one in the Grace's drawer. The other one, she tied it with a tope to a heavy stone, and after Grace had run away from the bridge, she shot herself. With Maria's being unable to pull the heavy gun anyomore, it fell to the river. With the help of another hook and piece of rope, Coventry finds the gun tied to a heavy stone. Maria wanted to frame Grace Dunbar, even if it cost her her life.
Watson is happy that Grace is released and proved innocent, and he's sure she will marry the American soon. Holmes says that Watson is too fast in absolving both of them, although the last images of Grace and Neil together, smiling to each other in a cab, lead me to think that it's implied that precisely that is what is going to happen.