This reads a bit like a prequel to the first two Moomins books I read. In those books, Moominpappa spends a lot of his time up in his study, writing his memoirs. In this book, the memoirs of his early life are recounted. In essence, he's reading them to Moomintroll, Sniff and Snuffkin, occasionally taking a break so we know they're there. Once in a while Momminmamma wanders into one of the breaks in the memoir reading.
Anyway, in his early years, Moominpappa (who is obviously not a pappa at that point) lives in a foundling house run by a Hemulen Aunt. She is rather bossy. He runs away and comes across Hodgkins, who views himself as being an inventor. Hodgkins builds a ship, and they sail off together, in company of the Muddler and the Joxer. It turns out that the Muddler and the Joxer are the fathers of Sniff and Snuffkin, so that maintains their interest in the adventures. Eventually, they arrive at the Autocrat's Island. The Autocrat hires Hodgkins as his royal inventor, or some such. Hodgkins settles down to rebuilding his boat, Oshun Oxstra (meant to be Ocean Orchestra, but the painter, Hodgkins nephew, the Muddler, didn't spell particularly well) as a flying boat. Moominpappa needs an occupation, so gathers up a ragtag bunch into a colony, The Royal Outlaw Colony. Well, as happens in Moomin books, new creatures (beings?) show up and are brought into the fold, including the Mymble's Daughter, The Mymble herself along with oodles of her children, including, eventually, Little My (who, I gather, becomes more prominent in later Moomin Books), and a ghost. Also the Fuzzy, who marries the Muddler, thus giving Sniff two parents. Toward the end, Moominpappa rescues Moominmamma from the sea and we've most of the group assembled for the previous adventures and the ones to come (presumably, I've not read ahead). There are, of course, additional adventurous activities going on and additional creatures (beings?) encountered. Yes, this menagerie is a bit confusing, but they're also charming and, for the most part, good natured and well meaning. We can use a bit of that these days. Oh, I should not forget that we also have quite a number of charming drawings of the characters in this book. The drawings really help to keep the disparate characters straight.