
15 plot points

Train platform. Feet walking. Guy Haines, tennis pro, and Bruno Anthony, stranger, board same train. Random encounter. Two men, different worlds. Innocence before corruption. Chance meeting will destroy lives.
Bruno: "Some people are better off dead." Theme of moral compromise, how easily good people can become complicit in evil, and danger of entertaining wrong ideas even hypothetically.
Bruno knows Guy from papers. Guy unhappily married to Miriam who refuses divorce. Guy loves Anne, Senator daughter. Bruno has controlling father he hates. Guy listens politely to stranger on train. Seems harmless conversation.
Bruno proposes "criss-cross" murders: "I kill your wife, you kill my father. We are strangers, no motive, perfect crime." Guy laughs it off, leaves. But Bruno is serious. Crazy person has plan. Guy should run but too polite.
Guy tries to divorce Miriam. She refuses, taunts him, says she is pregnant with other man child. Guy angry, storms off. Did he want her dead? Bruno follows Miriam to carnival. Should Guy have taken threat seriously? Too late.
Bruno strangles Miriam at carnival. Reflection in her glasses as she dies. Murder committed. Guy is alibi at time. Bruno calls Guy: "Your wife is dead, I did my part. Now you do yours." Guy trapped. Entering nightmare of complicity.
Guy and Anne relationship. She believes in him. But can he tell her truth? Police suspect Guy. Anne father Senator. Guy career and love threatened. If truth comes out about Bruno, loses everything. Trust tested.
Bruno stalks Guy. Appears at tennis matches, parties. Gives him gun, house plan. Demands Guy kill father. Guy refuses. Bruno threatens to frame him for Miriam murder. Promise: innocent man hunted by psychopath he cannot escape.
Bruno goes to Senator house, ingratiates himself. Anne sister Barbara looks like Miriam. Bruno at party, stares at Barbara, starts strangling woman while demonstrating, goes into trance. False victory: Guy thinks Bruno exposed himself. But makes him more dangerous.
Bruno plans to plant Guy lighter at murder scene to frame him. Guy must win tennis match quickly to stop Bruno. Racing against time. Police following Guy. Bruno going to carnival. Converging toward confrontation. Everything falling apart.
Guy rushes to carnival after tennis. Bruno already there planting lighter. Guy arrives, confronts Bruno on carousel. Fight. Police think Guy is attacking innocent man. Carousel speeds out of control. Trapped on death machine.
Carousel spinning wildly. Bruno and Guy fighting. Old man crawls under to stop it. Children screaming. Will crash. Bruno has lighter. Guy helpless. One stranger conversation led to wife murder and his own possible death. Nightmare peak.
Carousel crashes. Chaos. Bruno crushed, dying. Hand opens revealing lighter. Guy vindicated. Witness confirms Bruno was strangling Miriam. Truth revealed. Synthesis: innocence reclaimed through surviving evil he did not choose.
Bruno dies admitting everything. Police apologize to Guy. Free of suspicion. Can marry Anne. Career saved. But traumatized by experience. Innocent conversation with stranger nearly destroyed him. Learned terrible lesson about evil.
Guy and Anne on train. Stranger tries to talk. Guy and Anne immediately get up and leave. Will never talk to strangers again. Scarred by experience. Cautionary tale. Random encounter with madness. Haunted but free.