This is the fourth in Richard Osman's "Thursday Murder Club" series. I've been hooked from the first; it's a definite get-at-library for me. Newbies should start with #1; it's not something you want to jump in the middle of.
This entry is (I think) darker than its predecessors. I'm pretty sure the body count is higher. And there's a very serious and somber subplot; be warned that it's not all tea and cakes for the club members.
But the main plot thread is straightforward. An antiques dealer known to the club, Kuldesh Sharma, has a small but profitable sideline as a go-between for heroin dealers. It's been low-risk for a while… until it isn't. Not much of a spoiler: on page 2, he's killed by a gunshot to the head. The box containing the heroin has gone missing, however. Concerning both the people who gave it to Kuldesh, and the people who expected to get it from Kuldesh.
The core group from the Cooper's Chase retirement community are all here: Elizabeth the ex-spy; Joyce the ex-nurse; Ron the ex-labor activist; and Ibrahim the shrink. And the allies (and some antagonists) they've managed to pick up along the way: a couple of cops that have been bigfooted out of the investigation of Kuldesh's murder by the British equivalent of the FBI (I think).
An amusing subplot involves another resident, Mervyn, who has fallen in love with a Lithuanian lady on the Internet. Which involves him sending her a lot of money. It is completely obvious to everyone he's being scammed; can they rescue him and retrieve his cash? Yes, they can, as it turns out, and there's a cute interaction with their main case near the end.
Osman's next book doesn't involve the TMC, but he promises they'll return someday.