Spoiler alert: Do not read unless you’ve watched the penultimate episode of “Mad Men,” titled “The Milk & Honey Route” (episode 713).

It was inevitable, really.

Given all the cigarettes, all the smoking we’ve witnessed over the past seven seasons — someone would have to get lung cancer. We just all always thought it would be Don.

But it turned out to be Betty — and her chilling reaction to the diagnosis took everyone by surprise. (Shout out to this week’s director Matthew Weiner for the beautiful shot of the doctor giving the diagnosis to her husband, Henry, with Betty in the foreground.)

Her refusal to seek treatment frustrated Henry, who wanted to seek other opinions — but Betty was having none of it. So out of desperation, he sought out Sally, proving once again that she really could use at least one decent adult in her life. And that Kiernan Shipka has blossomed into one hell of an actress.

She barely has time to process the news: she goes through a roller coaster of emotions, she covers her ears. “It’s OK for you to cry, honey,” he tells her — then breaks down himself, forcing her to comfort him.

And when he brings her home, Sally expects to run into her mother’s welcoming arms — but true to WASP-y form, Betty storms right past her. So Sally takes her mother’s seat at the head of the table, folding toddler Gene into her arms.

When Betty comes to her room later, Sally snaps at her, “He (Henry) doesn’t know you won’t get treatment because you love the tragedy.” But Betty replies calmly, “I don’t want you to think I’m a quitter. I fought for plenty in my life and that’s how I know when it’s over. It’s not a weakness.”

And with that, she hands her daughter a letter to be read when she dies:

“I always worried about you because you marched to the beat of your own drum. But now I know that’s good. I know your life will be an adventure.”

Maybe she does know Sally after all. Cue the waterworks.

Meanwhile, Don’s off on an adventure of his own: a road trip through America (“Wyoming had a two-headed cow!”), which comes to a sudden stop in Oklahoma when his car breaks down. He’s stuck at a motel while he waits for the repair — and reluctantly accepts an invite to a fundraiser at a VFW. “I was in advertising,” he tells the motel owners. See ya, McCann.

The evening is loaded with dread — will he be exposed as Dick Whitman? There’s so much tension when he’s asked his rank, when a fellow Korean War veteran comes over to his table. “Let me see your face,” asks the vet. Shudder.

It’s almost a relief when Don’s secret comes out in a drunken exchange of war stories: “I killed my CO. We were under fire. Fuel was everywhere. I dropped my lighter. I blew him apart. And I got to come home.”

But the other vets barely react: “You just do what you have to do to come home,” they say.

Of course, Don doesn’t get away scot-free: Believing him to have stolen their stash, the vets visit him in the middle of the night and deliver a beating with a phone book. But Don knows who the real culprit is — the motel’s jack-of-all-trades, who was also working at the VFW.

When Don advises him to return the money, his message is loud and clear: “If you keep it, you’ll have to become somebody else, and it’s not what you think it is.” Still, he offers him a ride out of town — and then ultimately, the ride itself, handing over the keys to his Caddy, leaving him on the side of the road with his bag of the clothes.

The milk-and-honey route hasn’t quite worked out for Don. But it has for Pete, who manages to work a lucrative job offer out of Duck Phillips’s new client, Lear Jets. Even more impressively, Pete somehow worms his way back into ex-wife Trudy’s heart with an impassioned speech (“I’ve never loved anyone else. Ever.”).

For a quietly devastating episode, at least someone got a happy ending.

Quick thoughts:

• The episode opened with Don dreaming he got pulled over by a policeman. “You knew we’d catch up with you eventually.” Foreshadowing much?

• Ending with Buddy Holly’s “Everyday” was an eerie juxtaposition to the mood of the episode. So upbeat, yet hinting at more disaster to come.

• No Peggy. No Joan. No Roger. It’s hard not to have an episode with them, but Joan did have a shining moment last week. And with such knockout punches this week, one can only imagine what Weiner has in store for the finale.