SANTA CLARA — The plan, like so many plans in life, was a fanciful, intricate, and doomed.

The 49ers had it all mapped out. Their young defense, littered with baby faces, would undergo a baptism by fire. It would take its lumps, learn on the fly, and hopefully find its rhythm somewhere along the way. Meanwhile, the big four of the offense — Brock Purdy, George Kittle, Trent Williams, and Christian McCaffrey — would serve as a steady, high-octane crutch, grinding out wins and buying time. By late November, the two units would be humming in unison, a juggernaut no one wanted to face in January.

In-house, they called it the Rams Plan. The hope was it would work like the last few seasons in Los Angeles — that the slow, steady build would peak at just the right moment.

And they were convinced it was going to work.

Then came Week 1.

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All four of the Horsemen are on the injury report going into Sunday’s Week 2 game with the Saints. Purdy is out with a mangled toe and a bum shoulder, putting a wrench in everything. Kittle is on injured reserve, gone until at least Week 6. Williams, for the first time in forever, looked less like a human bulldozer and more like… a guy. And Christian McCaffrey? His health is a subject best discussed in hushed tones, lest you wake up those problematic calves.

The Rams Plan? Right now, the Niners are just trying to keep the season from falling apart before they even get to Week 5.

The way they’ll do that is simple: They’ll have to flip the script. It’s time for a bit of improv.

And, ready or not, it’s the defense’s turn to carry the load.

Robert Saleh is going to earn his salary, and then some this season, folks.

The 49ers’ defensive coordinator, back after a three-and-a-half-year sojourn trying to uncurse the Jets — a noble, if ultimately unsuccessful, endeavor. He was, without question, the team’s biggest offseason free-agent acquisition. (Not that there was much competition.) He has the complete trust of Kyle Shanahan and the organization as a whole, and that trust will be put to the test over the next couple of weeks.

But that trust looked fully justified in Week 1 against the Seahawks.

This was not the Saleh we last saw with the Niners in 2020 — rushing four, dropping seven into coverage, and playing zone look after zone look.

No, this was something different entirely. It was almost like an alter ego was calling the shots:

Hello, Bob Saleh.

“We’re not just lining up in the same spot like years past,” Saleh said.

Indeed, they are not.

Bob was getting funky with it in Seattle. Those five and six-man fronts that teams are using against Shanahan-prodigy offenses? Saleh added them to his bag, with middle linebacker and certified football madman Fred Warner spending a good deal of time in the A-gap.

Rookie defensive end Mykel Williams was used as a strong-side edge setter, a defensive tackle, and as an outside linebacker in coverage.

Rookie safety Marques Sigle? He was lined up everywhere and anywhere, blitzing from all over and dropping into areas Seahawks quarterback Sam Darnold did not foresee.

In all, it seemed as if Saleh mixed in more pass coverage looks in one game than Nick Sorensen did the entirety of last season.

(I won’t bore you with the details, but suffice it to say there was a Cover-1 Robber look involved.)

The Niners’ defense was, in a word, aggressive.

It had to be — there is only one way forward for this defense, and ergo, this team: through.

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The result? Seattle only scored 13 points, and the Niners graded out well in every meaningful advanced defensive metric.

“Showing those looks, making the O-line have to respect it… Not allowing them to protect the way they want to initially — that split second of indecision is how you create openings for guys [like me] on the edge,” Nick Bosa told me Wednesday.

Yes, there’s a method to the madness. Not only does it create opportunities for Bosa up front and Warner behind him, but it masks some of the defense’s deficiencies both in talent and experience.

“There are mistakes out there that we didn’t get called on,” Saleh said.

There’s no reason to think that won’t be the case moving forward. There are simply too many kids out there to trust the defense to line up and force an offense into submission.

Or, to butcher a cross-sport metaphor: This defense doesn’t have a dominating fastball like in 2019 (a four-man rush that got home time after time). No, the Niners need to go full junkballer.

So Saleh’s new approach can’t be a one-shot deal. This is the new normal, the way they have to live now if they want to keep the season from spinning off the rails.

But while the Niners had the element of surprise in Week 1, that won’t be the case moving forward. Saleh must dig deeper into the bag of tricks, finding more wrinkles, more deceptions, more junk, more funk week after week.

While an offense with backup quarterback Mac Jones at the helm might be a repetitive slog to watch for the next few games, this defense needs to be anything but predictable. It needs to keep them guessing.

The Rams Plan is dead. It’s time for the Bob Plan.

It might just keep the Niners in business this season.