Derrick Henry rushes for 75 yards and two touchdowns as Alabama bowls over Michigan State and into the national title game Jan. 11. (Tom Pennington/Getty Images)

Porous on the first night of 2015, Alabama proved inviolable on the last. Careless on the first, it proved airtight on the last. Inconvenienced with a pyrotechnic opponent on the first, it mastered a simpler puzzle on the last.

Beaten last Jan. 1 when it first tried one of these newfangled national semifinals, the Crimson Tide wound up absolutely not beaten on Friday night in the Cotton Bowl at AT&T Stadium, because it did what it tends to do in this near-dynastic era under Coach Nick Saban.

It improved systematically.

“The focus that they had for this game was completely different from what we’ve ever had before,” Saban said.

As it found its way to a 38-0 win over Michigan State and its fourth title-game berth in Saban’s nine seasons, set for Jan. 11 in Arizona opposite Clemson, Alabama did many of the things it didn’t and couldn’t 364 days prior in its 42-35 loss to eventual champion Ohio State. “To do that in a playoff, a goose egg up there?” hybrid front-seven man Ryan Anderson said of Michigan State’s score. “It’s a great feeling.”

Where it sprang leaks for the Buckeyes’ 537 total yards and 281 on the ground, it used its enviable and populous defensive front to make Michigan State look helpless. Where it allowed 85 rushing yards on a single clinching run by Ezekiel Elliott, it allowed less than 5 percent of that (4 yards) to four Spartans running backs across the telltale first three quarters. “It’s the first time all year that that’s happened to us,” Michigan State Coach Mark Dantonio said of his team’s rushing inability. And where Alabama threw three interceptions in New Orleans, one returned for a critical touchdown, it threw none here in Texas, and that statistic dovetailed with a surprising reality.

Alabama had the better quarterback — for the night, anyway.

For much of the Crimson Tide’s 12-1 season to date, Jake Coker had seemed at least something of a hood ornament, charged with handing the football efficiently and wisely to Heisman Trophy winner Derrick Henry. Yet with Michigan State bunched in to stop Henry — he gained 75 yards on 20 carries — Coker made precise throw after precise throw. With some damaging accuracy added to his usual efficiency, the senior first-year starter from Mobile, Ala., spent those first three quarters going a shining 25-for-30 for 286 yards that traded in game-changing moments.

With a stalemate underway and a scoreless game halfway through the second quarter, he lofted a beauty up the middle of the field toward a spot near the goal line, where soon there would appear Calvin Ridley, the onrushing freshman aiming to continue Alabama’s recent penchant for sublime receivers. Ridley ran beside defenders Demetrious Cox and Montae Nicholson until he turned up beyond them, collecting the 50-yard pass at the 1-yard line, from which Henry scored.

By the time Coker and Ridley connected on another pretty 50-thing late in the third quarter, a flawless touchdown pass that made the score 31-0, well, that wasn’t even necessary. Ridley would amass eight receptions for 138 yards, and Coker would say, “We’ve just got a lot of athletes on our side.”

To set up a 47-yard field goal for kicker Adam Griffith, Coker threw a pinpoint pass dripping just over a defender for 41 yards to tight end O.J. Howard, whose three receptions for 59 yards almost matched the four he had known across the previous five games. Griffith’s kick made it 10-0, a margin that looked wider because of how Alabama’s peerless defensive front was complicating the final mission of a veteran, 39-start quarterback.

That quarterback, Connor Cook, helped steer the Spartans to three surpassing seasons with two major bowl wins, two Big Ten titles and a lovely three-year record of 37-5: 13-1, 11-2 and now 12-2. It’s perhaps unfitting, then, that his last Michigan State game transpired in such hopeless muddle.

He often looked off on his throws, and certainly hassled by Alabama’s rotating band of ill-tempered trespassers up front — particularly Ryan Anderson and Jonathan Allen, the latter sliding through for two first-half sacks. “It’s a blessing to have those guys upfront, you know,” cornerback Cyrus Jones said. Cook finally did find the rhythmic end of his occasionally unpredictable game, and led Michigan State 63 yards in seven plays as the second quarter waned. Then he lofted a hopeful pass into the far-left end zone toward Aaron Burbridge, only to witness the leap of one of Alabama’s five-star athletes, Jones.

When Jones reached and then collected the interception, Cook had made the only graphic quarterback mistake of the game.

By the time Jones would help himself to a 57-yard punt return that made the score 24-0 in the third quarter, that, too, wasn’t even unnecessary.

The Big Ten champions had lived off gritty survivals and improbable wins at Michigan and Ohio State, but now they found themselves too overmatched to deploy any grit. The two-year-old College Football Playoff, meanwhile, had managed to feature four different conferences reaching the first two title games, so that Alabama fans did chant eventually: “SEC! SEC!” Saban effused praise on his players, saying they had set their own curfews and, “It wasn’t a bowl game to go and have a good time. It was, ‘We want to play well in this game.’ I certainly think we did that.”

An Alabama team that lost at home to Ole Miss on Sept. 19 had scrubbed away its foibles — five turnovers that night — and reached the final stage. Just as well, it had mended itself from New Year’s night to New Year’s Eve, enough to look as dominant as, well, Alabama.