Relatively Speaking by Alan Ayckbourn
Greg and Ginny are living together, but Greg is becoming somewhat suspicious that he is not the only man in her life. He wonders about Ginny's plan 'to visit her parents' and decides to follow her.
Ginny is really going to see a considerably older lover but only in order to break up her relationship with him. Greg mistakes the ex-lover and his wife for Ginny's parents.
Ginny's arrival further compounds an already wildly hilarious situation.