Nightswimming for Daydreamers (1970)

Two secret lovers, a disgruntled husband, a narcoleptic hitman, and the mysterious phone-operator pulling all the strings. Over the course of one increasingly booze fueled evening, the hitman trails the odd love birds on their final late night tryst. But this kill may not go so swimmingly, once the margaritas start flowing and a psychoactive toad is thrown into the nightcap cocktail.