My Summer of Love

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On a sunny summer day in Yorkshire Tamsin meets Mona. The former travels on horseback and chances upon Mona, who is resting in the grass. The two girls come from completely opposing background; Tamsin is from an upper-middle-class family and has recently been suspended from boarding school for being a bad influence; drinking alcohol. Mona, on the other hand, comes from a dysfunctional family, and her brother, Phil, who has recently been released from prison, is the only remaining member of her family. Both of the girls seem to regard their lives as mundane. The new acquaintances head home together, one on horseback and the other on an engineless scooter.
When Mona arrives home, she finds her brother in their mother's former pub, pouring all of the alcohol into the sink. He has undergone an extreme religious transformation during his stint in prison and regards his activity as part of his preparation for a rally for born-again Christian converts, much to Mona's annoyance. That night, Mona meets her lover, Ricky. They have sex in his car, after which he breaks up with her. Mona angrily turns down Ricky's offer to drive her home. The next day, Mona arrives at Tamsin's in order to escape from the rally organised by her brother. The girls begin to bond as they spend the day drinking and smoking while talking about the problems they face in their lives. Their sharing brings to light several of Tamsin's family secrets, such as the fact that Tamsin's sister, Sadie, died as a result of anorexia nervosa.
The next day, Tamsin takes Mona to the place where Tamsin claims that her father is cheating on her mother with his secretary. Not only is Tamsin furious because she states that the secretary is blonde with big breasts, but also because the secretary is not very smart. Mona smashes a window of Tamsin's father's car. The girls then flee the scene, after which Mona agrees to spend the night in Tamsin's house.
The next day, Tamsin purchases an engine for Mona's scooter, after which they drive to a small river to swim. While swimming under a waterfall, the girls share a kiss. The two return to Tamsin's house and Tamsin encourages Mona to try some of her old dresses. When Mona tries on a long red dress, Tamsin states that it used to belong to her sister, and tearfully recounts the process of watching Sadie die. She also tells Mona that she is beautiful in the dress. Later on, the two are in the back garden, with Tamsin playing Camille Saint-SaÃ«ns's cello solo Le Cygne from Le Carnaval des Animaux and Mona dancing The Dying Swan to it. Mona falls to the ground at the end of the piece, and Tamsin kisses her passionately. Later that night, the two girls have sex in Tamsin's bed, but are briefly interrupted by Phil, who has come to Tamsin's house to look for Mona.
The next day, Phil finds the girls sunbathing, with Tamsin topless. She does not cover herself and merely gazes at him as he invites the girls to his rally, in which he wants to erect a large cross on the hill next to their village. Although Mona is reluctant to go, Tamsin encourages her, and the two join the born-again Christians on their way up the hill. Tamsin behaves as if immediately attracted to Phil during the course of the rally. Later on, Mona and Tamsin explore Sadie's room and find a bag of magic mushrooms hidden inside a dollhouse. They take the psychedelic compounds together. Later that day, feeling the effects of the mushrooms, Mona and Tamsin go to a dancehall where they behave intimately, disturbing the mostly older patrons and prompting the bouncer to kick them out. They then go back to the river where they first kissed and declare their eternal love to each other, solemnly swearing that they would kill each other if one should leave the other, with Mona adding that she would kill herself.
In the morning, Tamsin wakes by the river cold and shivering, and the girls leave to breakfast at Tamsin's house. Phil then arrives at Tamsin's house searching for Mona, whereupon Tamsin pretends to seduce him. He reacts and attempts to kiss her, but she laughs him off insultingly, sparking his fury and violence, as he grabs Tamsin by the neck in a fit of anger. He proceeds to lock Mona in her room, forbidding her to see Tamsin, where Mona draws a picture of Tamsin on the wall. Mona fakes suicide and Phil comes in to rescue her, whereupon Mona mocks Phil's belief in the devil and he hits her across the face. Something in Phil snaps and he reverts to his violent behavior, kicking the born-again Christians out of the former pub, while Mona leaves the pub determined to start a new life with Tamsin.
Mona arrives at Tamsin's house and discovers that Tamsin is to return to boarding school. Mona also belatedly uncovers Tamsin's deception; Tasmin's parents were never separated for they were only on holliday out of the country, and Tamsin was never expelled from school for she was only on summer vacation. Also, Tamsin's older sister, Sadie, is in fact alive, and did not die of an eating disorder. Everything that Tamsin has said were lies fed to Mona as part of Tamsin's idea of summer fun. Mona, feeling disgusted and dejected by the fact that she's been used as summer theatrics, leaves for the girls' special spot at the river.
Tamsin finds her there and tells Mona that it should have been clear from the start that their relationship was doomed. In explaining why she lied about Sadie's "death" and about her father's infidelity, Tamsin tells Mona, "I'm a fantasist, for God's sake." Seemingly forgiving her, Mona slips into the water fully clothed, enticing Tamsin to join her, and the two kiss, re-enacting the earlier kissing scene. As they are kissing, Mona suddenly grabs Tamsin by the throat and pushes her under the water, as if about to fulfil her earlier oath to kill her. Instead she releases her and climbs out of the water. Despite Tamsin's angry shouts and insults, Mona confidently walks away.

Question: What was Mona's earlier oath to do to Tasmin?
Answer: Kill her