In The Mousetrap, newlyweds Giles and Mollie Ralston have inherited a large estate called Monkswell Manor. The action of the play takes place in its Great Hall. To generate some regular income, they've decided to convert the old house into an inn. The paying guests who arrive at the inn will become suspects in a perplexing murder mystery. But for now, Giles is more concerned about the success of his new business venture. He tells Mollie frankly that
We’re rather mugs at this game.
What he means by this is that the Ralstons are new to the hospitality business and so don't really know what they're doing. For her part, Mollie's a good deal more optimistic about their prospects of success, even though she appears to have a rather over-simplified notion as to what running an inn actually involves:
They bring luggage. If they don’t pay, we hang onto their luggage. It’s quite simple.