Resolution of recalcitrant nodes in the Tree of Life has been substantially eased recently by increased worldwide sampling and advancements in sequencing technology. It has become routine to use molecular data to characterise and taxonomically allocate tens to hundreds of taxa based on the DNA occurring in a few drops of water. Despite this, the adult stages of one invertebrate taxon, the enigmatic crustacean ‘y-larvae’ (Thecostraca: Facetotecta), have never been found, and the true diversity of this group has long remained unknown. Here, we review the current state of our knowledge concerning these mysterious larval forms and provide a significant body of new morphological and ecological data to make y-larvae accessible to a wider community of biologists. After summarising the history of y-larva studies, we review the current state of facetotectan systematics and outline in detail the structural features of each known phase in the life history, from y-nauplius through y-cypris to ypsigon. In particular, we document a suite of new ultrastructural details of the ypsigon, the putatively invasive phase if, as suspected, the adults are parasitic. Scanning and transmission electron microscopy are used to show that after a moult the ypsigon inherits numerous structures directly from the preceding y-cypris larva, including parts of the nervous system, sensory organs, pores, and antennal musculature. A comparison of the ypsigon to the very similar, yet surely independently evolved and thus merely analogous invasive stages of parasitic barnacles (Rhizocephala) provides a broader phylogenetic and functional context for these findings. These structural traits agree with molecular phylogenetic data placing Facetotecta as an early- branching sister clade to the cirripede barnacles, including Rhizocephala. We then review the ecology and biogeography of y-larvae in the global plankton and offer a comprehensive map and list of recorded localities. Finally, we statistically test the abundance of the different life-history stages of y-larvae in the surface plankton at one of our primary study sites (Sesoko Island, Okinawa, Japan) in relation to various environmental factors that may drive their occurrence there. We found evidence of crepuscular emergence around dawn (7 am-9 am) and dusk (5 pm-7 pm). This review is the most comprehensive synthesis of information on Facetotecta to date. Despite our continued ignorance of the adults, we hope it will serve both as a starting point for future scientists embarking on studies of this challenging group of crustaceans and as an inspiration for those working on other kinds of planktonic larvae.