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David J. Phillip, AP
Kansas City Chiefs quarterback Alex Smith throws against the Houston Texans on Sunday, Oct. 8, 2017, in Houston.

SALT LAKE CITY — It’s difficult to miss the irony. Alex Smith is off to one of the best starts in NFL history while Colin Kaepernick, once the future of quarterbacking, is out of football just five years after chasing Smith out of San Francisco.

They were the central characters — along with Jim Harbaugh — in a drama that was followed like a soap opera by American sports fans. None has emerged so well, and so gracefully, as Smith.

Kaepernick’s play began to decline the year after he replaced Smith once opponents learned to slow-play the read option, and he lost his starting job and eventually his employment. The temperamental Harbaugh wore out his welcome and fled to the college game. Now Kaepernick is doing whatever it is fulltime political activists do, and Harbaugh is trying to figure out how to beat Smith’s old college coach, Urban Meyer, at Ohio State.

Playing well is the best revenge and Smith is doing all of that. The Kansas City Chiefs are the only unbeaten team in the NFL, and through five games Smith has been brilliant. He leads the NFL in completion percentage (76.6), pass efficiency rating (125.8) and yards per attempt (8.8). He has thrown for 1,391 yards, 11 touchdowns and 0 interceptions.

According to Pro-Football-Reference.com, Smith has had the fourth-best start in NFL history through five games, trailing only Peyton Manning, Tom Brady and Daunte Culpepper. Smith is the only one among the top 10 who had no interceptions, and he ranks first in completion percentage.

Alex Smith is the Story of the Year so far in the NFL, and if you have any heart at all you’re pulling for him.

Ever since the University of Utah quarterback was made the first pick of the 2005 NFL draft, it seems everything — fate, coaches, injuries — conspired to sweep him aside or write him off. He played poorly during his first few years in the league and the 49ers were largely at fault.

As noted here many times, the 49ers completely mishandled their young quarterback. He had seven offensive coordinators and four head coaches in his first seven seasons. He was thrust into the starting lineup early in his rookie season when neither he nor the under-talented offense was equipped for it.

It was a recipe for failure. Two of his head coaches publicly questioned his toughness and manhood. One of them scolded him on the sideline like a school child on national TV, prompting broadcaster John Madden to say, “That’s really not part of coaching.”

When Smith had an injured shoulder that he claimed hampered his ability to throw, one head coach doubted publicly that he was really injured. Smith wound up undergoing surgery and missed part of one season and all of the 2008 season.

Smith’s career was going nowhere. He was relegated at various times to a backup role behind J.T. O'Sullivan, Shaun Hill and Troy Smith. In his first three seasons he had 11 wins, 19 touchdown passes and 31 interceptions. He was battered and broken, physically and mentally; he was widely called a bust.

The 49ers were ready to cut their losses with Smith, but then along came Harbaugh in 2011. He talked Smith into returning to the team and revived his career only to turn his back on him. Smith and Harbaugh won 13 of 16 games and advanced to the conference championship game and were on their way to the Super Bowl until the team’s punt returner lost a pair of fumbles.

In 2012, Smith was among the leading passers in the league and the Niners were 6-2-1 when he was sidelined by a concussion. Kaepernick replaced him and when Smith was healthy again Harbaugh elected to stay with the new guy even though Smith had a 19-5-1 record in his previous 21 games. The Niners went on to the Super Bowl and lost to the Ravens; a year later Smith was traded to the Chiefs.

Andy Reid, the Chiefs head coach, has embraced Smith, but last spring he paved the way for his replacement. Smith has never been the big-play quarterback. He doesn’t throw downfield much and the Chiefs have always relied on a sound running game.

Smith is often referred to as a “game manager” — a backhanded compliment that means he won’t win games, but he won’t hurt you either. There was some truth in that. Remarkably, Smith has only eight 300-yard games in his 13-year career; Brady and Aaron Rodgers both have nine of them since the beginning of last season.

After the 2016 season — in which the Chiefs won the division title but failed to reach the conference championship game — the Chiefs traded up to get the 10th pick of the draft, which they used to select quarterback Patrick Mahomes. Once again, Smith was being swept aside. Even he conceded that Mahomes was the Chiefs quarterback of the future and that this was likely his last season with the Chiefs.

Meanwhile, Smith has been the best quarterback in the league. He’s also 27-9 in his last 36 games, averaging 10 wins a season in four-plus seasons with KC. As usual, he has handled the arrival of his heir apparent with aplomb.

By all accounts, there is none of the tension that accompanied the arrival of Rodgers on Brett Favre’s Packers or Steve Young’s arrival on Joe Montana’s 49ers. Nor did he ask to be traded, which is what Sam Bradford did when the Eagles drafted Carson Wentz. It’s been business as usual for Smith, and that’s good news for the Chiefs.