Knives Out

Harlan discovered that Marta had accidentally given him a lethal overdose of morphine due to misreading the medication labels. Rather than let her be investigated, which would threaten her undocumented mother's deportation status, Harlan chose to kill himself and stage it as a suicide to protect her. He also used this moment to ensure his death would appear as murder, shielding Marta from suspicion.

Earlier, Harlan jokingly warns about confusing real and fake knives while placing a display knife into his game stand, hinting at the deadly misunderstanding to come.

Harlan amended his will to leave everything to Marta because she was the only family member who genuinely cared for him without ulterior motives. He distrusted his greedy relatives and wanted to reward her honesty and compassion. The will was changed shortly before his death, shocking the family who expected to inherit his fortune.

Ransom switched the labels on the morphine and its counteragent, planning to cause Marta to overdose a patient and be blamed for murder. When that plan failed because Harlan protected Marta, Ransom switched to Plan B: tampering with evidence, burning the medical examiner's office to destroy toxicology reports, and orchestrating a conspiracy to make Marta look guilty of murder.

Ransom's heated argument with Harlan where he threatened "I'm warning you" foreshadows his involvement in the murder plot.

Ransom's motive was greed and entitlement. When he learned Harlan had amended the will to leave everything to Marta instead of the family, Ransom felt robbed of his expected inheritance. He threatened Harlan and orchestrated the murder plot to ensure he'd receive his share of the estate.

Ransom was conspicuously absent during the investigations and had a prior conflict with Harlan over the family's inheritance expectations.

Marta has a physiological condition where she vomits whenever she tells a lie. This becomes a crucial plot device throughout the investigation, as Blanc uses it to detect her deception. Her inability to lie cleanly forces her to be mostly honest, which ultimately helps Blanc trust her and solve the case.

Blanc methodically reconstructed the timeline by questioning each family member and analyzing the physical evidence. He noticed Marta's vomiting when lying, which helped separate her truthful accounts from others' deceptions. He discovered the tampered security footage, analyzed the muddy footprints, and eventually uncovered Ransom's conspiracy through his thorough investigation and understanding of everyone's motives.

Blanc's unusual method of having a stranger play a piano note to unsettle witnesses shows his unconventional investigative style.

Fran was Ransom's accomplice who helped him tamper with evidence. She overdosed from a morphine vial Ransom had stolen, but she survived. She later threatened Ransom with a copy of the toxicology report, leading him to burn down the medical examiner's office. In the end, Marta saved Fran from an abandoned building where Ransom had left her for dead.

Harlan tripped Marta to prevent her from calling 911 or leaving immediately after discovering his overdose. By causing her to fall, he forced her to stay in the room longer, giving him time to explain his plan to protect her through a staged suicide. He needed her to leave quietly through the trellis to establish an alibi.

The blank letter was part of Harlan's deception. He had promised Richard a letter exposing his affair to Linda, but instead gave Richard a blank letter. This was Harlan's way of giving Richard a second chance, either confess honestly or live with the fear of exposure. Richard found relief in the blank letter, not knowing it was intentional.

Marta's undocumented status made her vulnerable to deportation if she became a murder suspect. Harlan was aware of this and specifically chose to protect her by staging his death as a suicide rather than let her be investigated. This fear of authorities also motivated Marta to go along with Harlan's plan, as any investigation could lead to her mother's deportation.

The investigation exposed several family secrets: Richard's extramarital affair (which Harlan had photographed), Walt's plan to fire his son Jacob from the publishing company due to his alt-right trolling, and Joni's financial dependence on the family. These secrets revealed the deep dysfunction and hidden conflicts within the Thrombey family.