Spoiler warning: Spoilers for the complete film or season named below.
Grace and Juan Carlos are the killers, with different grievances against Malcolm Quince. Murder Mystery deliberately offers a solution that seems complete before one remaining death forces Nick and Audrey to keep investigating.
What is Grace hiding about her family?
She is Quince’s daughter, using a different identity from the one connected to her birth. Her place in the family gives her a financial motive and a grievance against the father who abandoned her.
The inheritance question makes that identity more than a surprise about ancestry. It changes who stands to benefit from Quince’s death and why his plans for his money threaten the people gathered around him.
Nick and Audrey reach the solution while trying to clear themselves during a holiday that has become an investigation. Their position outside the wealthy group lets them question the relationships its members present as settled facts.
Why isn’t identifying Grace enough?
Vikram points out that she could not have killed Sergei. A theory that explains one motive and one set of clues still fails if it cannot account for a specific death.
Juan Carlos supplied another part of the plan. He believes Quince caused the racing accident that cost his father his legs, giving him a motive independent of Grace’s claim to the inheritance.
The partnership brings money and revenge together without making those desires identical. The killers can cooperate because the same man stands between each of them and what they want.
How does Juan Carlos’ escape end?
He takes Inspector de la Croix hostage and drives away. Nick and Audrey pursue him, and a tour bus ultimately kills him. It is the excursion they were originally supposed to join, brought back into the story at the least ordinary moment of their trip.
The couple then continue the holiday with a journey on the Orient Express. The reference places their experience inside the familiar tradition of murder-mystery travel without adding another solved case.
Their success depends on refusing to stop once a plausible culprit has been named. Grace’s secret is important, but plausibility is not the same as an explanation that fits every event.
The last chase answers the practical danger after the second reveal. The final train ride returns Nick and Audrey to being a couple on vacation, now sharing confidence in what they managed to work out together.
Sources: official ending breakdown; official interview or additional context.