Plum wine (or umeshu, as it is known in Japanese) is made by steeping Japanese ume plums in sake alcohol and sugar until the flavour of the plum has infused into the alcohol.
This particular combination of ingredients gives umeshu a deep, sweet/sour flavour. Umeshu is always better served chilled, but it can be enjoyed straight, on the rocks, or combined with other beverages for a fruity cocktail.