The Innocents

Miss Giddens (Deborah Kerr) applies for a job as a governess. It is to be her first position, but the wealthy bachelor interviewing her (Michael Redgrave) is unconcerned with her lack of experience. He values his freedom to travel and socialise and unabashedly confesses that he has "no room, mentally or emotionally" for his niece and nephew. They were orphaned and left in his care as infants, and he keeps them at Bly, his country estate. The previous governess, Miss Jessel, died suddenly less than a year ago. All he cares about is that Miss Giddens accept full responsibility for the children, never troubling him with whatever problems may arise.
At Bly, Miss Giddens is instantly taken with Flora. She also forges a friendship with Mrs. Grose (Megs Jenkins), the kindly housekeeper. The boy, Miles, is away at boarding school, though Flora delightedly insists that her brother is coming home. Miss Giddens then receives a letter saying that Miles has been expelled from school because of his bad influence on the other boys. Mrs. Grose says she can't imagine Miles being a bad influence, and when Miss Giddens meets the boy herself, she too thinks his teachers must have exaggerated. He seems charming and mature â though perhaps too mature, with flirtatious flattery toward his governess.
The children are friendly and polite, but Miss Giddens is disturbed by their occasional odd behaviours. They seem to be sharing secrets. She is upset by unexplained voices and by several visions of a woman and man, whom Mrs. Grose identifies, from their descriptions, as Miss Jessel and Peter Quint â the uncle's valet until his death. Eventually, Mrs. Grose reveals that Quint was abusive to Miss Jessel, and that they were indiscreet, performing sexual acts in plain sight of the other servants and even, perhaps, the children. After Quint's death, Miss Jessel went into a deep depression and drowned herself.
When Miles recites a poem invoking a "lost lord" to rise from the grave, Miss Giddens concludes that the ghosts of Quint and Miss Jessel possess the bodies of the children so they can continue their relationship. She is determined to rescue them from this possession. One night Miss Giddens finds Flora at a window, watching Miles, who is walking in the garden. When Miss Giddens escorts him to bed, he kisses her goodnight, in a disturbingly adult manner.
The next day Miss Giddens finds Flora dancing alone by the lake â and again sees the figure of Miss Jessel staring at them from across the water. Convinced that the children will be freed from the possession if they will confess what is happening, Miss Giddens begs Flora to admit that Miss Jessel is there. Flora begins to scream and cry, calling Miss Giddens wicked and insane. Hours later, Flora is still hysterical, and when Mrs. Grose finally leaves her bedside, she says she can't imagine where Flora learned such obscenities. Miss Giddens orders her to take Flora away from Bly. She is certain that Miles is on the brink of confessing his ordeal to her and that she must be left alone with him.
That night, alone with Miles, Miss Giddens presses him to talk about the ghosts, and then about why he was expelled from school. Initially, and as usual, Miles is glib and evasive, but he eventually admits that he frightened the other boys with violence and vulgar language. Miss Giddens enjoins him to say who taught him this language and behaviour. Miles suddenly begins yelling obscene insults and laughing maniacally, and Peter Quint's face appears in the window behind him, joining in the boy's laughter. Miles then runs outside; Miss Giddens follows, calling that all he has to do is "say his name" and it will all be over. Quint appears on the hedge nearby, but Miles does not seem to see him and screams that she is insane. He finally shouts Quint's name, then the hand of Quint appears. Miles grows still and falls to the ground. Miss Giddens cradles him and assures him that he is free. She then realises that Miles is dead. Sobbing in horror, she leans over him and kisses him on the lips.

Question: Who is alone by the lake?
Answer: Flora