As I flew out to California last week to drive the 2017 Alfa Romeo Giulia Quadrifoglio at Sonoma Raceway, I could hardly fit my skepticism in the overhead. We are in Year Seven of Fiat Chrysler Automobiles ’ CEO Sergio Marchionne’s Five-Year Plan. This is the part where the Italian brands Maserati and Alfa Romeo are transformed into profit-spewing global luxury brands, in the mold of Porsche. Molto fortuna, Bubba.

But give Mr. Marchionne credit. The colossal machine he has built is moving. FCA’s growing nimbleness on the product side has helped it catch the luxury-SUV wave at a propitious crest. Just arriving in your local Nordstrom’s parking lot this fall, and taking up two spaces, is the new Maserati Levante SUV ($72,000), which looks enough like a Cayenne to make you sneeze. The Levante has the potential to double Mazzer’s North American sales, which is always a winning strategy.

2017 Alfa Romeo Giulia Quadrifoglio Interior of the 2017 Alfa Romeo Giulia Quadrifoglio. Photo: Alfa Romeo Base price: $70,000 (est.)

$70,000 (est.) Powertrain: front-mounted, twin-turbocharged, direct-injected 2.9-liter DOHC V6 with variable boost, variable cam phasing, cylinder deactivation and stop-start; eight-speed automatic transmission; rear-wheel drive with torque vectoring

front-mounted, twin-turbocharged, direct-injected 2.9-liter DOHC V6 with variable boost, variable cam phasing, cylinder deactivation and stop-start; eight-speed automatic transmission; rear-wheel drive with torque vectoring Power/torque: 505 hp at 6,500 rpm/443 pound-feet at 2,000-4,800 rpm

505 hp at 6,500 rpm/443 pound-feet at 2,000-4,800 rpm Length/weight: 182.6 inches/3,800 pounds

182.6 inches/3,800 pounds Wheelbase: 111.0 inches

111.0 inches EPA fuel economy: 16/24 mpg, city/highway (est.)

16/24 mpg, city/highway (est.) Trunk volume: 14 cubic feet (est.)

And now it’s Alfa Romeo’s turn in FCA’s premium/luxury-brand sausage grinder. The company has invested five billion euros into a total wipe and reboot of Alfa Romeo, including: a freshly automated assembly hall in Cassino, Italy; a new lightweight, front-engine/rear-drive (or AWD) architecture; and a brace of wee turbo’ed engines, also designed in-house, with some borrowed talent from Ferrari.

The Giulia debuts the brand’s voluptuous form language, a blown-glass bottle from which decants Italian masculinity. The sedan silhouette is conservative but the face, the historic hawk-bill grille, is nicely dramatic, modernized with a dull-gleaming V, for veloce? The Quadrifoglio is skirted-round with carbon aero bits—including an active front splitter—and the five-lobed alloy wheels look amazing, especially wrapped in a haze of Pirelli smoke.

2017 Alfa Romeo Guilia Quadrifoglio Photo: Alfa Romeo

The new Alfa brand means to be smart, tech-intensive and aspirational, with dominant genes of gorgeousness descended from Italian couture and industrial design. With the new RWD/AWD powertrains, Alfa can again be taken seriously by enthusiasts.

Most important, Alfa is being reborn as an honest-to-God volume auto maker, offering mass-class on a scale that will make old-time Alfistis blow out their spark plugs. The Giulia sedan is only the first of a reported six vehicles to be built on the new Giorgio architecture, including two crossover/SUVs. And no matter how fine the Giulia sport sedan turns out to be (absolutely fantastic, by the way), everyone knows Alfa’s long-term health depends on the bucks-up SUV and crossover models to follow.

Mr. Marchionne has said he would like to see Alfa Romeo brand sales in the neighborhood of 400,000 units annually, by the end of 2018, a mere eightfold increase over 2015. He also asked for a pony.

For these reasons and more, the Giulia project seemed pretty unpromising: Here’s yet another historic European marque, being reinvented out of thin air and exploited to sell luxury crossovers to people who wouldn’t know Ascari from Campari. Thanks, Fiat.

But then I drove the Giulia, and it absolutely shut my mouth. Far from irrelevant, the Giulia Quadrifoglio turns out to be instantly indispensable, a must-drive: certainly the lightest- and liveliest-handling, the most poised, pointable and neutral sport-sedan I’ve ever laid hands to. I love the steering wheel so much I’m thinking of having my palms upholstered in faux suede.

Here’s a quick sketch: midsize, five seats, majority-steel structure, powered by a twin-turbo 2.9-liter V6 (505 hp, 443 lb-ft between 2,000-4,800 rpm); eight-speed automatic (no manual transmission); and rear-wheel drive with fully articulated torque vectoring. The Quadrifoglio has a curb weight of 3,800 pounds, with 50/50 weight distribution, front/rear. On the strength of its class-leading weight-to-power ratio (7.5 pounds per hp) the Quadrifoglio snarls to 60 mph in 3.8 seconds, and boasts a reported top speed of 191 mph.

Those are big numbers. Here’s a bigger one: The Alfa Q ship laid down a blazing 7:32 minute lap time at the Nürburgring, a record for a series production four-door sedan.

You might wonder how such an overachieving car emerged from a milieu that has so often rewarded mediocrity. I sure did. It certainly helped that the stakes were so high. Legend has it Mr. Marchionne himself sent the early Giulia prototypes back to the drawing board, declaring them uncompetitive with the German competition.

That decision set the Giulia, the entire Alfa reboot back many months. In the end, Alfa was obliged to rebuild its own silo, so to speak. For example: The Giorgio architecture was developed in-house and purpose-built for Alfa Romeo. This is a costly bit of kit. While the passenger cell is made of steel, the front and rear sub-frames, suspension and brake componentry are all aluminum, as are the doors and fenders. The drive shaft is made of carbon fiber.

The Quadrifoglio adds lightness with a carbon-fiber roof and hood, as well as optional carbon-ceramic brakes, which are 50% lighter than iron rotors. Why not? It’s only money.

Alfa also developed its own engines: The Quadrifoglio’s bi-turbo, 2.9-liter, 90-degree V6 is an evolution of Ferrari’s turbo V8, from valvetrain design to cylinder bores. At full throttle between the corners, the Quadrifoglio climbs Sonoma’s short, steep hills like they weren’t even there.

Interior of the 2017 Alfa Romeo Giulia Quadrifoglio. Photo: Alfa Romeo

Next year Alfa will start shipping the Giulia Ti, powered by a 2.0-liter, 280-hp four cylinder, an engine also of the company’s own design. I was able to put some miles on the Giulia Ti on the roads around Napa, and, on balance, this would be the trim level I recommend. The Ti is plenty athletic for real-world driving, while the touring tires are quieter, ride better and offer more security than the Quadrifoglio’s Pirelli P Zero Corsa gumballs.

Actually, on a cool, damp Sonoma Raceway, the Pirellis were not really working that great. Under heavy braking the front tires skittered and hunted for grip, and the car pushed wide and got loose before the stability-control intervened.

Until l get this car on up-to-temp tires, I’ll remain officially skeptical. That way maybe they will send me another one. But the Quadrifoglio absolutely shakes my faith in faithlessness. All of a sudden, I want to believe in Alfa Romeo.

Corrections & Amplifications:

The 2017 Alfa Romeo Giulia does not have a dual-clutch transmission. An earlier version of this article incorrectly stated that the car has this feature.