It starts with a monster truck show, literally: more zombies than we've ever seen outside of Brad Pitt's World War Z corralled into a quarry that's blocked at its two exits by strategically parked semi-trailers. Weird. And there's Rick (Andrew Lincoln), high above, barking orders. This much, at least, is familiar.

He's got little plaster strips all over his face like some beefcake Norman Gunston; it can't be too long after he and Pete beat the crap out of each other in last season's finale, in a fight that might have been considered a dead heat had Rick not had the final word by pulling out his heat and shooting Pete-the-abusive-alcoholic (Corey Brill) dead. Sure, mayor Deanna (Tovah Feldshuh) had given him permission with those brittle words "Do it" after Pete had accidentally chopped her husband's neck in half, but Rick didn't really need much urging.

So, here we are, and he's back in boss mode. Is he still unhinged? Or is he hatching a plan to save the docile flock of Alexandria?

It sounds like he's got a plan, though it's hard to make sense of it in these opening scenes. This is a dry run, he says, for the real exercise planned for tomorrow. But when one of the trucks gets pushed off the road by the shuffling horde – seriously, it looks like the queue for the toilet at a Taylor Swift concert – all bets are off. The walkers are on the move. "We do this now," Rick shouts.

I think he may have said that before, somewhere. And somewhere else.

Suddenly we're in black and white mode; it's earlier. We can tell this because while Morgan Jones (Lennie James) is eating a bowl of cereal, Rick's face is still bloody-pulp. He tells Morgan – whom we first met way back in season one and who has been shadowing Rick and his band for a while now – that he no longer takes chances. "I don't blame you," says Morgan, whose sole weapon of survival these days is a broomstick. If that ain't taking a chance in the age of the zombocalypse, I don't know what is.

What Rick is getting at is you can finish your Rice Crispies and get the f--- outta my house. There's a fixer-upper across the street you can stay in – locked in a room – for a while. Don't you love that southern hospitality?

But we know what's going on. We saw that look Morgan gave when he stumbled into town just as Good Cop Rick was blowing Pete's brains out. Disapproving. Disappointed. "Dis ain't you."

That's why Rick doesn't want him around. If Pinocchio had been able to send Jiminy Cricket to someone else's shoulder, he would have, right?

Anyway, this back and forth between colour and black and white is the big stylistic trick director (and lead effects guru) Greg Nicotero makes in this episode, and while it's kind of clunky it's also a Godsend; those time shifts would be pretty damned confusing otherwise, unless he somehow contrived to have Rick covered in Petebrain for all of the "before" sequences. Hmmm; maybe next time.

The plan, it evolves, is to guide the walkers out of the quarry and away from Alexandria. If we don't, Rick says, they're going to come straight here.

The reason the town has been spared all these years isn't because it has solar power or grey-water treatment or ridiculously high real estate values ("Homes from $800,000" says the fading sign on the road) – go ahead and high-five yourselves, climate change deniers – but because the walkers were drawn to the quarry by the noise of the other walkers, and then couldn't get out. A bit like Friday night clubbers stuck in some place they'd rather not be after the 1am lock-out.

The plan is simple, though it unfolds in tiny little black-and-white slivers, which makes it seem complicated. Darryl (Norman Reedus) will ride slowly out front, making enough noise with his hog that the zombs will have no choice but to follow. Sasha (Sonequa Martin-Green) and Abraham (Michael Cudlitz) volunteer to ride shotgun in a car, which gives Abraham the opportunity to ask Sasha if she's only doing this because she wants to be killed. "Doing something like this, it's living," she says. Well sure, unless you get caught.

Back in town, Jessie (Alexandra Breckenridge) seems to have gone cool on Rick, which is a bit odd because you'd think killing a woman's husband was a sure-fire way to get her attention.

There's some B&W dissent from the townies too, about the plan to make Alexandria safe by leading a squillion zombies past its front door. Carter (Ethan Embry) thinks it's mad. He thinks Rick should be killed before he gets them killed. He almost shoots Eugene (Josh McDermitt) in the mullet when he catches him spying on a gathering of rebels. But if anyone has noticed Carter before now, speak up. If not, you know how this is going to end. He's going to die.

I wonder how the agent sells a gig like this to an actor. "Hey, man. I've got something great for you. The Walking Dead. It's a big part. You get to be an asshole for 43 minutes then you have your face bitten off and everyone thinks, 'Yay'. Whaddyasay?"

"Hell yeah."

Anyway, Carter the unstoppable squawking machine might be a pain, but he's right. As the horde is corralled past the walls that the Alexandrians have thrown up overnight (well, maybe a couple of nights, if Rick has been changing his sticky plasters; it's hard to tell) the plan seems to be working. And then it stops. There's some godawful blaring of a horn, and it's coming from Alexandria. The walkers are drawn to it.

The episode ends with a chopper shot of thousands of zombies moving through the forest on the outskirts of town, inexorably drawn towards its feeble-bodied and unprepared inhabitants with just one thing on their minds: to stop that horn or we're calling the police.

Bloody hell.