Reeling from the loss of their eldest son to the war, a family welcomes the company of a mysterious soldier claiming to fulfill a promise to a fallen comrade.
This sounds very similar to the plot of The Guest.
I would recommend having one protagonist whose perspective we see this story from. Give us the logline from this character’s perspective.
Inciting incident – what is the incident that throws the protagonist’s life upside down? I’m assuming that, since it’s a thriller, something dramatic happens that kick starts the real story.
Goal – when we have a protagonist, give them a clear visual goal that is their answer to the question posed by the inciting incident.