SPOILER ALERT: Stop reading if you have not seen the Nov. 29 episode of “Homeland.”

Finally, nine episodes into “Homeland’s” fifth season, we start to understand the reason for the rift between Carrie and Saul.

There was a lot of other activity in “The Litvinov Ruse” — and still more ripped-from-the-headlines material involving ISIS and Europe’s home-grown terrorism crisis — but this hour is rooted in a relationship that has been the core of “Homeland” from the beginning. The master and the protegé. The sensei and the student. Only this time, the tables are turned for Saul and Carrie.

“Litvinov Ruse” features a story by Howard Gordon and Patrick Harbinson, teleplay by Alex Gansa and is directed by Tucker Gates. It opens with Etai, the Mossad agent who has come to Saul’s aid in his time of need in Berlin, bringing Carrie to Saul’s hideout in an artist’s loft. Etai is characteristically blunt with Carrie before she hops out of the car. Apparently after the debacle that was Islamabad two years ago (in season four), Carrie lobbied against Saul getting a promotion to head of the CIA — a job that went to his friend and rival Dar Adal. Maybe Carrie felt Saul was too worn out for all the responsibility, remembering his breakdown on the tarmac in season four, for one thing.

“You broke his heart, you know,” Etai tells Carrie, sounding like Saul’s brother from another mother. “Both our hearts got broken,” she responds. “His is older, weaker,” he fires back. Etai clearly gets through because Carrie can’t control her quiver when she comes face to face with Saul a minute later. Saul is ever the forgiving dad. “C’mere,” he says, giving her a bear hug.

From here on out, however, Carrie is leading the charge — let’s face it, she’s essentially working for the CIA again — while Saul is put through the emotional wringer. Carrie has the unpleasant task of telling Saul, who heads the CIA’s European operations, that his Berlin station chief, Allison, is a Russian mole. And this is before she knows that Saul’s been canoodling with her. (That seems an un-Saul-like move but perhaps more evidence of his broken heart.) Who knew he had such feelings for Allison? Apparently it was more than a rebound relationship after Mira left him again. But Saul’s not a lovesick fool. Even as he resists Carrie’s theory about Allison, he knows she’s right when she insists that he can’t afford not to investigate to determine the truth.

Mandy Patinkin is so good it hurts throughout this episode. Especially when he’s come to bid Allison farewell, under the ruse of his intention of defecting to Israel to avoid the storm brewing against him within the CIA. “I was asleep for ten years. You woke me up,” Saul tells Allison. Does he mean it? Yes. Is he working her? Yes. Damn. He smooth-talks his way into her bed in order to plant a bug in her purse and a tracking device on her phone. As Lou Rawls would say, “Love is a hurtin’ thing.”

A shout-out is also due to the sound editing and mixing team on this episode. The scene with Saul planting the device in Allison’s purse builds crazy tension thanks to the clicks, taps and tink-tinks that come from the tiny tools that he uses to complete the job. The frame of Saul working on her purse in the dark included part of a staircase — just enough to make you hold your breath waiting for Allison to come charging down to find him.

As for the larger Allison sting operation in this episode, it feels like a push that Saul and Carrie, both of whom are under big-time suspicion, can go straight to top German intelligence agents to set up the elaborate net to catch the CIA station chief. But this thread gives us a big dose of the fantastic Nina Hoss as German agent Astrid so I’m not complaining.

Saul is later humiliated when he realizes what a nymph-o Allison is while he and Carrie and the German intelligence team track Allison’s movements. Carrie reminds Saul that she’s been through the hell of being intimately involved with a compromised source. “I’ve been there Saul as you well know,” she tells him. Ever the pro, Carrie also presses Saul on whether he whispered a warning to Allison. He knows that she has to ask. They go back to the hunt and pretty quickly catch their prey.

Miranda Otto delivers another terrific performance as Allison, the ice queen who schemes even as the walls are closing in on all sides. She works well under pressure — such as that applied by the hand of her Russian lover/handler Ivan wrapped around her throat. While he wigs out, she’s brainstorming a plan to paint Ivan as the double agent. She does it well enough to raise some doubts in the interrogation room in the closing moments of the episode.

As all this plays out above ground, in train stations, palatial estates and CIA conference rooms, Quinn spends the hour in duct tape, holed up with the rest of the eager jihadists in a big abandoned building. This season of “Homeland” has been so enriched by its visually compelling locations. Shooting Berlin for Berlin has brought a certain aufregung to the show this year.

Quinn figures out that the jihadis plan to use him as a sacrificial lamb to up the stakes on their threat to unleash a sarin gas attack on Berlin. Quinn, the man forever in the shadows, is led to the makeshift gas chamber with the blinding light of a video camera in his face and the promise that he will be all over TV and the Internet when Bibi and the gang release the video of him succumbing to sarin gas as a warning shot to the infidels.

Quinn keeps working on the nerves of Qasim (not sure on spelling), the one man in the group where he senses a ray of compassion. The two engage in a verbal tennis-match exchange that is chilling in light of the Nov. 13 terror attacks in Paris. Qasim’s quick replies sound like the kind of things that would have been drilled into young men in Terrorism 101 class.

Quinn presses Qasim about mounting an attack in his native country, because after all he is German. Qasim rejects out of hand because he’s never felt like he belonged there.

Quinn: Innocent people will be hurt by a sarin gas attack?

Qasim: The U.S. hurts innocents by sending soldiers to occupy Middle Eastern countries. “Terror is the necessary prologue to a caliphate,” he says flatly.

Quinn: What do you want?

Qasim: “Assad out of Syria and recognition by the U.N. of the Islamic State.”

Quinn: “That will never happen.”

Qasim: “Whatever happens it is Allah’s will.”

Quinn: “Maybe it’s Allah’s will that you stop it.”

But Quinn gets to Qasim on some level. Qasim manages to tackle Quinn just long enough to inject him with the antidote that might save him from the sarin gas exposure. (In all honestly, the scene with Qasim telling Quinn to “run” while others are easily within earshot pushes the credibility needle a bit.)

The last three minutes of the episode are among the most graphic “Homeland” has ever offered — right up there with Brody swinging from the gallows. Quinn convulsing and foaming at the mouth is not an easy sight to see.

I ended this hour getting mad at Carrie all over again for abandoning Quinn a few episodes back. She’s only got three more hours this season to make it right.