Spoiler alert: Do not read until you’ve watched episode two of Season 6 of “Homeland,” titled “The Man in the Basement.”

Carrie Mathison (Claire Danes) is about to have a very bad day — and it starts and ends heartbreakingly with the man living in her basement apartment, Quinn (Rupert Friend).

He’s not the only one who’ll cross paths with her over the course of this taut hour (written by Chip Johannessen, and directed by Keith Gordon). By episode’s end, her relationship with the other man in her life she once loved, Saul Berenson (Mandy Patinkin), will also be forever changed.

Let’s start with Saul. Their ever-complicated bond has had its ups and downs, to be sure — but when they’re first reunited here, they’re cautiously friendly. Past grudges seem to have been put aside as they greet each other in Carrie’s law office, where Saul has arrived for a surprise visit.

But Saul has an agenda: He’s come to probe whether his former protegee is pulling strings behind his back with the new presidential administration.

“I was thinking how long it had been since I heard your voice…And then I thought I did, when I briefed the president-elect on our covert action programs,” he tells her. “I think her entire national security platform came out of that head of yours.”

Carrie acts shocked at his accusations. She claims to be busy with her cases. Do we believe her? Does he? Hard to tell in that moment. But then he goes too far: “It would be a huge embarrassment to everyone if it got out you were a secret adviser to the president. That would not play well at all.”

Oh, Saul. Do you know nothing about Carrie? With that, she’s had enough: She kicks him out of her office with an angry flare of righteous indignation. That probably was her tell. Yet he believes her, as we’ll later learn. But he’s never been good at reading the women in his life. (See Allison Carr, Season 5.) When he meets up with Dar Adal later, he tells the spymaster confidently Carrie is not advising the PEOTUS. “She’s a menace,” snaps Adal about Carrie. Counters Saul: “Just not at the moment to us.”

But Adal knows better: He’s just seen the photo evidence of Carrie being screened by the Secret Service. Yet he, too, has his own agenda — securing his own position in the new administration, perhaps? — and he doesn’t reveal the truth to Saul. Saul won’t take kindly to being lied to, but more to come on that, I’m sure. Everyone’s lying to him: Adal, Carrie. Wonder which betrayal will sting more deeply.

Prescient as ever, this season of “Homeland” echoes the bitter distrust between the president-elect and the intelligence community, charting the tense days during the transition of power between administrations. PEOTUS Elizabeth Keane (Elizabeth Marvel), we learn, has secretly recruited Carrie as her counselor to advise her on issues of national security. No doubt she needs her: Her chief of staff is being fed a narrative by Adal that she’s suspicious of, about Iran cheating on the nuclear arms deal. Carrie nominates her old boss, Saul, to play intermediary — which only leads Adal to be more suspicious of him. As we learned last week, he’s already been keeping him from key meetings. And we’re still not entirely certain of Keane’s own true agenda: We know she’s mourning the loss of her son in Iraq, but there’s got to be more to her politics.

Meanwhile, Carrie’s defense case isn’t going well. Her efforts to free Sekou Bah (J. Mallory McCree), who stands accused of attempting to incite terrorism, only serve to illustrate the Catch-22 of the legal system. The more she digs, the further the quicksand collapses around her. She discovers the government’s case is built around a confidential informant who gave Sekou the money and romanced his sister. “If I can’t say anything and he’s lying, how am I supposed to get out of here?” says an exasperated Sekou, encapsulating the problem. He can’t understand what he did wrong: Where’s the line between free speech and inciting terrorism? Yet the government is convinced they’ve captured a potential terrorist, and they’re not yielding.

Yet Carrie’s facing an even bigger landmine at home. Her daughter, Frannie, announces innocently to her mother, “There’s a man in the basement!” And that man — Quinn, wounded, hallucinating, and frankly, rank — is haunting indeed, with a powerful performance from Friend. He’s not showering (“Something down here doesn’t smell very good: I think it might be you,” he’s told), obsessing over canned food, and listening to talk radio that’s fueling the rage building inside. Carrie appoints Max as babysitter, but despite Quinn’s clear physical issues — he escapes and has a seizure in a nearby bodega (he’s been skipping his anti-seizure meds) — Max sees the real problem: Quinn’s “strange thing” (as he calls it) with Carrie. “Why are you giving her such a hard time?” he asks Quinn. Quinn can’t, won’t answer.

But when Carrie gets home, Quinn’s ready with a question: “What happened?” She doesn’t understand at first, then realizes he means what happened to him — how did he end up so damaged. So eerily echoing her bedtime visit to her daughter, Frannie, whom she’d just checked in on minutes earlier, Carrie and Quinn sit side-by-side on Quinn’s bed, watching the nightmarish video where he was gassed — him for the first time, Carrie for the 100th. She narrates it clinically as the CIA operative, explaining how the tiles were the clue that ultimately led her to find him. He nods, silently. It’s the worst bedtime story ever.

Then he asks one more question — one that devastates her: “Why?” It’s such a loaded question. (Cue that famous quivering lip.) He’s asking why she saved him. It won’t be — it can’t be — long before he learns that it was under her orders that he was revived from the coma. And that was the true cause of so much of his injuries.

The stage is set for some sparks to come — not the good kind.

• One note I just had to point out:

When Marvel’s chief of staff, Rob (Hill Harper) is summoned for a lunch meeting with Dar Adal, he tries to stand up to him but isn’t quite up the task, so he has to endure a monologue from Adal about rescuing people on 9/11 at the very restaurant they’re in. But when Adal and Saul meet up later at the same eatery, Saul can’t resist teasing him: “Still telling the bullshit story about dragging people in here when the towers came down?” Well played, Dar.