WHAT IT’S ABOUT Bill Masters (Michael Sheen) and Virginia Johnson (Lizzy Caplan) are a couple no more, after their split in the third season. She’s finding her own way in the world, and he is trying to cope — with booze as a crutch — somewhere himself. Bill’s wife, Libby (Caitlin FitzGerald) is done with him as well. So, yes — it’s grim, and just as their Reproduction Biology Research Center is overrun with patients and fame. Even office receptionist Betty DiMello (Annaleigh Ashford) has lost her patience.

In Sunday’s fourth-season opener, Hugh Hefner (John Gleeson Connolly) makes his entrance, and Libby finds empowerment — sort of — in a feminist group led by Anita (Alysia Reiner of “Orange Is the New Black.”) Bill finds a guardian angel, Louise (Niecy Nash). The new season picks up in 1968.

MY SAY By the start of the fourth season, Ginny and Bill have liberated themselves from the confinement of “conventional” relationships, and unconventional ones, too (their own) only to confine themselves in other ways. They’re ’60s-era swingers — whose research helped revolutionize swinging — except they don’t particularly like to swing themselves. They’re fractured and a mess, and even “Hef” tells them to get their act together: “Do therapy on yourself,” he advises. You know you’re in trouble when Hugh Hefner starts dispensing relationship advice.

That’s where the fourth season (entitled, appropriately, “Freefall”) finds Masters and Johnson. They’re apart, but still in some sense bound together. They can’t do anything without affecting the other in some way — their curse, and still perhaps, their promise.

“Masters of Sex” never found all that much humor in their complicated emotional predicament, but this opener does. Even Bill — holding up a bar stool and tumbler of whiskey — invites some measure of parody. And when Ginny pays a call on Hefner — a voluptuary in a bathrobe and slippers, who is high-minded and low-minded in the same instance — then parody is a given.

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The fourth season is a critical one for “Masters.” Ratings are low and buzz has evaporated. At least the opener indicates this remains an intelligent series in search of complex answers to complicated questions, like “What is love?” Or, Will Bill and Virginia ever find it — either together or apart?

BOTTOM LINE A lighter touch and a welcome one, too.