I've set up that straw man to push back at it: not to push back against Lowry being a drunk; I don't know, but the evidence seems strong; but rather against it being simple, against it being a roman á clef, against it being unmediated portrait of the day-to-day existence of Malcolm Lowry.
The story goes something like this: Geoffrey Firmin, the (ex-)British consul in the Mexican town of Quauhnahuac, is drinking himself to death. The majority of the novel takes place on the Day of the Dead, November 2nd, 1938, though the first chapter takes place a year later and in it, we learn that Firmin is dead. In the second chapter, Firmin wakes up hungover after a party the night before, and he's in search of (and in need of) the hair of (some, many) dogs. His half-brother Hugh is due back that same day; Geoffrey Firmin's ex-wife Yvonne, estranged because of his excessive drinking, also arrives, unexpectedly, with the idea that maybe things can still be patched up. How he gets through that day (and then doesn't) is the story.
Yvonne seems serious in her wish to patch up things with Geoffrey, but she's also clearly attracted to Geoffrey's younger half-brother. She and Hugh may have had an affair in the past. She's certainly had an affair with Laruelle, Geoffrey's best friend, a French filmmaker, but that occurs only after she's given up on Geoffrey because of his drinking.
Not a lot happens that day. Hugh and Yvonne go horseback riding. The main trio run into Laruelle. Early that evening Geoffrey, Hugh, and Yvonne go to a bull-riding event that's a bust because the bull doesn't buck enough. And then Geoffrey's final end. That's pretty much it. Most of the pages of the novel are devoted to unwinding how we got here.
One thing that's not resolved is why Geoffrey's a drunk. It's sometimes implied that Geoffrey's a drunk because Yvonne has had an affair, maybe more than one, but we learn Geoffrey was a drunk before that; maybe he's a drunk because of the trauma he went through as a naval officer in World War I, but we know he was drinking earlier; maybe it's due to the complications of his Anglo-Indian childhood and orphanhood? But he had opportunities after that; he developed a close relationship with his foster father. Geoffrey writes, and is working on a book--contributing to that feeling of a roman á clef--and could it be the stress of a creative career that sends him to the bottle? Well, in the end, we're not told, and it sort of doesn't matter: by the time of main events, Geoffrey Firmin is already a lost person.
It's the unwinding, the revelation of the backstory, I found fairly sophisticated and interesting. It's where the drama in the book is. The chapters are generally from the point of view of one character, largely rotating among those main three: Geoffrey, Hugh, and Yvonne. At some point in each chapter, the character enters into fugue state, half remembrance, half fantasy. So, during the boring part of the bull-riding, Yvonne's mind drifts off and we learn about her acting career, a successful enough star of second-tier Westerns as a teenager, a withdrawal from Hollywood, then ending with a hope and dream of returning that doesn't pan out. Elsewhere she also dreams of leaving Mexico with Geoffrey and going to Canada with him sober and working on his book.
In other chapters we learn about Hugh's career as a reporter, and his dream of being the hero of the saved Spanish Republic, just as (in 1938) Franco's forces are about to win conclusively.
But it's Geoffrey's memories and fantasies that are the most problematic, the most confused. And this, very subtly, is how we learn that Geoffrey really is lost. In person, people like him: Yvonne wants to reconcile; he has friends still, despite his compulsive drinking, who care about him; they think him capable of reform. But in his fantasy life, his thoughts are substantially past-directed; few of his thoughts touch upon that work he might write, and those confused; a few upon the renewed life he might build with Yvonne; but mostly they revolve around where that next drink is coming from. Hopes and fantasies may be unrealistic, but without them you're dead.
It's a bleak novel to be sure. But there's hope in it, too, just not much for Geoffrey Firmin.
The good news is Malcolm Lowry was at least partly Hugh, and not entirely Geoffrey: he was Hugh's age, not Geoffrey's; he did escape to Canada with the woman he loved; he doesn't seem to have ever escaped his alcohol problem, but he did write a novel that shows up on the Modern Library list of great 20th Century novels as well as other lists of great novels.
The edition I read is shown above and has an introduction by Stephen Spender. I thought Spender's essay was pretty good--he compares and contrasts Under the Volcano with works of high modernism like Ulysses or The Waste Land, which this book both is and isn't--but it should have been an afterword. He gives too much away. We know from almost the start that Geoffrey Firmin dies, but there's a bit of a mystery about the details, and those should have been left mysterious. If you read this edition, save his introduction until after.
Anyway, this one does multiple duty for me: it's on my Classics Club list, my #20BooksOfSummer list, my Back to the Classics list, and counts for my Canadian Book Challenge. Whew!