Restaurants waste one fifth of the food they buy; much is left behind on people's plates (Engström & Carlsson-Kanyama, 2004). Plate waste forces restaurants to buy more food than necessary, and to pay for food waste disposal; and burdens the environment, with agriculture “a dominant force behind…climate change” (Foley et al., 2011, p.337).Plate waste is under-researched in tourism (Gössling & Peeters, 2015), with buffets being particularly wasteful (15 g per person per day at breakfast; Juvan, Grün, & Dolnicar, 2018). Only two measures are proven to reduce plate waste at hotel buffets: smaller plates, and explaining to guests that getting seconds is better than taking too much at once (Kallbekken & Sælen, 2013). Social science theories underlie these effective practical measures: the idea of plate size reduction stems from obesity theory; and inviting patrons back leverages social norms. The present study develops a new “mini-theory of action” (Rossiter, 2011, p.119) to inform the development a wider range of practical measures for plate waste reduction. Mini-theories of action formulate possible reasons for human behaviour, opening these hypothesized cause-and-effect relationships up for empirical testing in real behavioural contexts (Rossiter, 2011). Mini-theories of action, arguably, are the starting point for any new social science theory. As such, our study's contribution is primarily theoretical. Pathways to practical impact are discussed.We conducted unstructured interviews with 10 chefs and 23 waiters/waitresses (of varying gender and work experience) in three four-star rated hotels serving buffet-style meals to 300–1200 guests daily. We asked why guests leave uneaten food behind. Service staff observe and talk to guests daily, frequently checking if they are finished. In response, guests offer explanations why some food remains uneaten. Chefs design menus to minimize food waste. We identify specific drivers of plate waste, and seven overarching themes.