You'll need a family tree to keep the characters straight, but it's worth it.
This reminds me of an earlier Miss Silver book - The Clock Strikes Twelve. I spent the first couple of chapters of that one trying to untangle the family relationships, but it turned out to be a good mystery and so is this one. The characters are (mostly) descendants of Jeremiah Travener, who owned and ran a seaside inn called The Catherine Wheel. He died in 1888, leaving six sons and two daughters. Needless to say, they had children and their offspring had children.
The Traveners don't go in for family reunions, so almost none of them know each other. Elderly, wealthy Jacob Travener decides to change that and he advertises to find some of his relatives, especially those of the younger generation. He claims to be looking for one worthy of being the heir (or heiress) to his fortune, but why is he only interested in those who grew up close to their grandparents - i.e. old Jeremiah's children?
The Catherine Wheel was passed to his grandfather and is now his. Tradition is that the inn was the center of a thriving ring of smugglers in the 19th century. Smugglers bringing contraband in by the sea normally use caves and secret passages to get illegal goods into a safe building where it can be hidden. Does the old inn have secrets that would be of value to modern smugglers?
Whatever his reason, Jacob invites eight relatives to be his guests at the inn for the weekend, promising them a substantial gift of money for accepting his hospitaltiy. All but one are eager to accept. WWII is not long over and the English economy is still struggling. The money will be welcome, especially with the prospects of a fortune in the future.
As would be expected, some of the family lines have risen in society and some have fallen. One is an aristocrat - Lady Marian. She's a charmer, but her attempts to marry into money keep falling short. The youngest relatives are firmly upper class, a career military officer and his cousin who makes her living modeling designer clothes. They are the only ones who were previously acquainted and (naturally) one of them is in love with the other.
The lower middle class is represented by the Travener bother and sister. He's an enthusiastic seller of household appliances and she owns a small shop. And there's brassy Florence Duke, who's risen from serving drinks at a pub to owning a snack bar. Al Miller and John Higgins are working class men. Miller lives near the inn, but is gathering with the family. Higgins is a carpenter on a local estate and a lay preacher. He's the only relative who refused old Jacob's bribe. He dislikes the current manager of the Catherine Wheel and chooses not to enter the premises. However, he's courting the pretty maid at the inn and is involved in the family weekend, like it or not.
What none of them know is that Scotland Yard is very much interested in the inn and what might be going on there. They know of Jacob's family get-together and want someone inside to see what goes on. And who better to choose than a harmless old lady? Inspector Frank Abbott will be nearby, staying with one of HIS many relatives. Superintendent Lamb groans that Abbott produces relatives like a magician pulling rabbits out of a hat. This particular rabbit is a baronet who's ancestral mansion has been connected to the Catherine Wheel Inn in some peculiar ways.
Miss Silver worms her way into a room at the inn and shortly afterwards there's (what else?) a murder. The murdered man is a stinker, but Scotland Yard can't let that stand in their way. Besides, he's suspected of being involved in the illegal activity at the inn and his murder may mean that his fellow smugglers have fallen out.
I think this is a very good Miss Silver mystery. The local inspector isn't a fool nor is he incompetent, but his sense of humor is poorly developed. He's not thrilled that Scotland Yard is involved and an elderly governess-turned-detective sitting in on police conferences and (worse!) making suggestions is a thorn in his side. He sees it as an open-and-shut case, but Miss Silver is quietly unimpressed.
Instead of accepting one of several logical suspects for the murder, she wants to know what happened to Al Miller and why the police can't find him. No sooner do the police figure out the whereabouts of Al Miller than another person disappears and the need to find that Secret Room becomes urgent.
Some of the characters are appealing and some aren't, but all are believable. I had my suspicions about one of the relatives, but was never sure and would not have been surprised if any of them was the guilty party. There are two sets of lovers, but the romance never gets in the way of the story and I was happy for all of them. Miss Silver will have new photographs of happy young couples to display in her office/parlor and (perhaps) baby booties to knit in good time. I love this series.