All depend on Thrombey for their financial well-being, and all have been threatened with ruin. Thus, a complicated web of intrigue and betrayal needs to be untangled in order to rule out foul play as a cause of death. As Thrombey’s nurse and only real confidante, Marta was the last person to see him alive. She also has a solid alibi, having been seen leaving the house after her shift by several family members. But most importantly, Blanc loftily points out, Marta has a weakness that others do not have: She cannot lie. And not in the George Washington chopping down the cherry tree honour system sense. Marta literally cannot lie; the stress of doing so, even in the most minor way, causes her to projectile vomit, a theory proven several times throughout the film. In other words, she’s the perfect person to help sift through the family’s secrets.