The Big Cube (1968)

Renowned stage actress Adriana Roman, the muse of playwright Frederick Lansdale, is retiring to marry tycoon Charles Winthrop. The marriage does not sit well with Charles' daughter Lisa, who dislikes her new stepmother if only because she is "the other woman," competition for her father's affections. However, on her father's urging, Lisa tries to get to know Adriana and slowly warms to her. Meanwhile, Lisa attracts the attention of Johnny Allen, a womanizing, fortune-hunting medical student who uses the school lab to produce and sell acid among his hip, mod crowd. He romances naive Lisa, who falls under his spell. After an incident with Charles, Johnny senses an opportunity to abscond with at least some of the Winthrop fortune by using Lisa's mistrust of Adriana, and he manipulates Lisa into trying to make Adriana go crazy, mostly by lacing her sedatives with acid, and Adriana starts to believe that Lisa and Johnny are trying to kill her. In reality, only Johnny is trying to kill Adriana; Lisa is completely unaware. As Adriana tries to explain to others what she's going through, Frederick, who has always loved her himself, eventually believes that all is not that it appears. With new information at hand, Frederick, using the tools of his trade, tries to help Adriana through her troubled mental state.