What is at the root of the Broncos’ Jekyll-and-Hyde offense?

ENGLEWOOD, Colo. — There are two Broncos teams, featuring two Denver offenses.

One started 0-4, with an 0-2 mark in games decided by seven or fewer points and an average point differential of minus-6 points per game.

The other has been a .500 team since then, with a 3-2 record in games decided by seven or fewer points and an average point differential of minus-2 points per game.

Despite starting three different quarterbacks, the Broncos have normalized as a middle-of-the-road team, with their offense and defense roughly canceling each other out.

But within the Broncos of the last half-season of work dating back to their Week 5 win over the Los Angeles Chargers, there are two versions of the Denver offense.

There’s the one that you see in the first quarter.

And there’s what you witness for the three that follow.

IN THE FIRST QUARTER …

The Broncos are among the NFL’s best, as evidenced by their place in a slew of metrics since Week 5.

• Total offense: 4th
• Points: T-5th
• Touchdowns: T-5th
• Passing yardage: 4th
• Rushing yardage: 8th
• Yards per play: 8th
• Yards per pass play: 7th
• Sack rate: 12th
• Yards per rush: 17th
• Plays that gain 20+ yards: T-2nd
• First downs: T-6th
• First-down rate: 12th
• Third-down success rate: 8th

Across the board, the Broncos have an efficient offense early in games. Much of this arises from the prepared script of offensive coordinator Rich Scangarello.

Like many offensive play-callers on branches that have sprung from the Bill Walsh tree, Scangarello lays out the first 12 to 20 plays in a script that is a focal point of practice and preparation.

As Walsh noted in his 1997 book Finding the Winning Edge, the script helps coaches diagnose the tendencies of opposing defenses and helps the players because “the procedure enables them to know ahead of time what they will be doing on their first one or two series,” wrote Walsh and co-authors James Peterson and Brian Billick (yes, THAT Billick, the coach who led Baltimore to a Super Bowl XXXV win).

“You kind of piece it together over a four- or five-day process,” Scangarello explained in October, “so that when you get to Friday it’s all come together and you pretty much have a real solid foundation for the stuff you want to do to kind of put the defense on their heels or get after them on a fast start.”

Whether Joe Flacco, Brandon Allen or Drew Lock has been the quarterback, the Broncos’ script has worked. Even in Buffalo, the Broncos didn’t go three-and-out in the first half … but closed the game with five consecutive three-and-outs.

SO WHAT ABOUT THE REST OF THE GAME?

Now look at the Broncos’ place in those statistics since Week 5 for the second through fourth quarters:

• Total offense: 32nd
• Points: 32nd
• Touchdowns: 32nd
• Passing yardage: 32nd
• Rushing yardage: 17th
• Yards per play: 32nd
• Yards per pass play: 32nd
• Sack rate: 30th
• Yards per rush: 14th
• Plays that gain 20+ yards: 32nd
• First downs: 32nd
• First-down rate: 32nd
• Third-down success rate: 32nd

Is it play-calling? In terms of the type of passes, perhaps; the Broncos emphasized throws near, at or behind the line of scrimmage last week.

But it’s not about how often the Broncos run the ball. In the first quarter of games since Week 5, the Broncos run on 46.5 percent of their snaps, the 10th-highest percentage in the league, per statistics and and gamebooks from NFLGSIS.com. In the second through fourth quarters, they run 44.4 percent of the time, a rate that ranks 12th.

“I think it’s more about execution,” Scangarello said Thursday, speaking about last Sunday’s Chargers game. “It’s not an aggressiveness issue. You look at the play calls [in] the second half, we threw it more than we ran it. I think we were 10 pass, nine run.”

So in the Broncos’ eyes, they see the discrepancy between their form as coming down to one issue:

THIRD DOWNS

When asked about the difference between the early production and what followed, Vic Fangio pointed there.

Last Sunday, the Broncos were 5-of-6 on third downs in the first quarter and since Week 5, the Broncos have converted 48.2 percent of their first-quarter third downs, better than 24 teams.

But in the final three quarters of games since Week 5, the Broncos have converted just 15.8 percent of their third downs.

Not only are the Broncos at the bottom of the league in this category in the final three quarters since Week 5, they’re at the bottom by a margin of nearly nine full percentage points; the No. 31 team is Washington (24.56 percent).

Often, third-down failures can be attributed to first- and second-down shortcomings. Usually a low third-down conversion percentage stems from facing too many third-and-long situations.

But according to pro-football-reference.com, the Broncos rank 15th in the amount of yards they need to get on third down over the last half-season: 7.4.

From those plays, the Broncos have averaged 2.94 yards per snap, next-to-last in the league — and in the final three quarters of games, just 1.95 yards, the fewest in the league. The Broncos are also next-to-last in second-down yardage (4.62) and are last in the final three quarters (4.14) of games since Week 5. But they rank 11th on first downs (5.74 yards per plat) and in the final three quarters of games, they are a respectable 17th on first downs, averaging 5.37 yards per play in that time span.

“The good teams don’t even get to many third downs,” Fangio said. “They get a first down on first or second down.”

But when the Broncos get to third down, nothing has been worse than their work on third-and-5-or-fewer yards.

Per a league-wide play analysis, since Week 5, teams have converted an average of 52.9 percent of their third downs when they needed 5 or fewer yards. The Broncos have converted just 32.6 percent of their chances, including only 20.6 percent (7-of-34) of their opportunities in the final three quarters since Week 5. Both of these are the lowest figures in the league.

“We can’t miss third-and-1s and third-and-2s; that’s what stopped us in the second half in half of our drives,” Fangio said. “We’ve got to be able to convert those.”

Each third down tells a different story — a dropped pass here, a missed block there, a damaging penalty. The Broncos are using every color in the feel-bad palette of offensive football.

“There are some times where plays are called and we don’t execute the things we need to do to keep the chains moving — you know, it’s third-and-2 and we don’t convert, or it’s third-and-5 and we don’t convert it,” explained left tackle Garett Bolles.

“Those are the downs that you’ve got to convert to keep the chains moving and put some points up on the board.”

The Broncos haven’t done that.

Much of this reveals an offense that is still young and in flux. Third-down issues are the symptom. Quarterback instability, a developing offensive line and a lack of a vertical-speed threat at wide receiver are among the root causes.

Nevertheless, with what the Broncos possess, there have been pockets of success — and they have come in nearly every game.

But if Scangarello and his players can’t figure out how to take their early-game scripted success and make it work off-script, their pattern of lost leads and blown opportunities will continue.