There's a sense of foreboding that hangs over Season 3 of Catastrophe. That might seem obvious given the show's title, but Season 3, which drops on Amazon Prime on Friday, features one of the final performances by Carrie Fisher, who filmed her appearance days before her fatal heart attack in December.

But even if that cruel twist of fate hadn't happened, Catastrophe hurdles down a darker path in its third season, one that's at times unflinchingly uncomfortable, but still with its trademark bawdy, ferocious humor intact.

Unlike the sophomore season, which skipped three years after Season 1, the third season picks up seconds after the Season 2 finale, in which Rob (Rob Delaney) found the receipt for the morning-after pill Sharon (Sharon Horgan) purchased after a one-night stand with a young musician. Or was it? She's not exactly sure what had happened and sets out to get to the bottom of it. (Spoiler alert: It's not full-fledged cheating, but it's still frowned upon.)

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Like many of their issues, the plot point, though it does reverberate, is quickly resolved and the confrontation is quickly, hilariously, painfully had. Sharon blames her ill decision-making on stress and tough times — including "Brexit, your new president" — and asks if Rob wants to break up. "Do I want to break up with you? What are we, 14?" he replies incredulously.

Catastrophe's greatest asset is its verisimilitude. Sh-- happens, and while it'd be easy to go for maximum drama, Catastrophe has always gone for maximum honesty. Joy, laughs and tears spring from Rob and Sharon's stumbles, their successes, their open wounds and their witty, playful and absolutely filthy banter. They might be TV's best couple. The show never makes us forget that relationships take work. You don't break up or walk out just because you can. And it continuously reinforces that the best relationships are the ones where you can lay into each other one minute and genuinely crack each other up the next. Nothing's off limits, nothing's withheld.

Sharon Horgan and Rob Delaney, CatastropheSharon Horgan and Rob Delaney, Catastrophe

Except that there is something this time. After losing his job over a sexual harassment claim last season, Rob fell off the wagon, and is well off of it as the couple's financial issues, a result of his job hunt futility, progress in Season 3. He masks it by rubbing junk food all over himself — a classic alcoholic cover-up move that goes over Sharon's head.

Rob's alcoholism has felt like a missile the show was going to deploy at some point (Delaney, a recovering alcoholic, has candidly talked about his own struggles), and his descent — a potent mix of misery, frustration and guilt — is hard and heartbreaking to watch. He knows he should stop, seeing what the bottle has done to Dave (Daniel Lapaine), but can't. He has a burning desire to tell Sharon, but can't. Is this the thing that's finally going to do them in? That's the fear, but Horgan and Delaney's writing (they penned all six episodes once again) is too nuanced to subscribe to conventional tropes.

It all comes to a head in the finale, which features Fisher reprising her role as Rob's cantankerous mother Mia. Ironically, she's back to attend a funeral. It's loaded and sad, but Fisher is as fabulous and cutting as she ever was, especially during a revealing mother-son conversation. Fisher won't be in Star Wars: Episode IX, but it's just as unfortunate that we'll never see Mia again and deepen that character and that relationship.

The season concludes with another cliffhanger, the show's biggest one yet. It's not an argument, but an incident that slowly builds suspense with the air of a horror movie before suddenly unloading. It sets up a fourth season, already ordered, that could really go anywhere.

Grief, Rob tells Sharon at one point, has no playbook. Neither do relationships, but Horgan and Delaney are writing one that's the most realistic yet.

Season 3 of Catastrophe will be released on Amazon Prime on Friday.