PHOENIX — If someone high in the Arizona sky were looking down through the rarely open Chase Field roof, entrusted with script-writing duties for the baseball game that was played there Sunday night, that person would have been laughed off the clouds for suggesting the plot that unfolded.

Because forgotten slugger Mark Reynolds did not just homer in his Washington debut at the home park of the team that drafted him when Nationals General Manager Mike Rizzo was the scouting director here. Reynolds homered twice, the second time in a tie game in the eighth inning to help the Nationals beat the Arizona Diamondbacks, 6-4, and complete a four-game sweep of a team that had not lost a series this season.

The Nationals have won 13 of their past 15 games. They are tied with the Atlanta Braves for the most wins in the National League East with 24. They went 6-1 on a road trip that was supposed to be more challenging than this, and now head into a week at home that includes two days off — treasures, by baseball standards.

That trip ended with a relatively late Sunday start. For the second time in two West Coast swings, the Nationals played on “Sunday Night Baseball,” which meant playing on a slightly bigger stage — if getting home much later — than they normally would on getaway day. But the late start allowed for a beautiful setting, as the Diamondbacks opened the Chase Field roof in the fifth inning. The ball tends to fly when the roof is open. It had already flown with the roof closed.

Bryce Harper hit a ball 468 feet at 115 mph in the third inning, and Trea Turner hooked a ball over the left field fence as the roof was opening in the fifth. By that time, the Nationals led 3-1. As has happened before this trip and in two starts on it, Nationals Manager Dave Martinez held starter Jeremy Hellickson to two times through the order, even though he had thrown just 61 pitches through five innings when Martinez took him out. Hellickson now owns a 2.20 ERA and a 0.86 WHIP (walks plus hits per inning) in six starts as a National.

Then up came Reynolds, who admitted he was nervous for his first at-bat, which resulted in a single through the left side that calmed the 11-year veteran. That he played here for four years didn’t hurt. His last big league game, with the Colorado Rockies in last year’s playoffs, came at Chase Field, too. He was so comfortable, in fact, that he followed that single with a home run in the sixth inning that upped the Nationals’ lead to three runs.

“I’ve had a lot of at-bats in this place,” Reynolds said. “. . . It feels like home. It’s a pretty comfy, pretty comfortable place to play.

But in the seventh, Nationals reliever Brandon Kintzler, who had allowed one run in his last 16 innings entering Sunday’s game, surrendered three to even it up 4-4. That brought Reynolds up with a tie game in the eighth. He turned around Archie Bradley’s high-powered fastball to untie it.

“As a player, you know how hard it is to play from game to game, from at-bat to at-bat. So many things can change,” Turner said. “To do it twice in one game and not play the last few days, I think it’s pretty unbelievable, pretty spectacular.”

Rizzo drafted Reynolds 476th overall when he was the scouting director for the Diamondbacks in 2004, meaning Reynolds joins Max Scherzer as players Rizzo selected during his tenure with Arizona who are now on the Nationals’ active roster. Reynolds played at the University of Virginia with Ryan Zimmerman, and he missed closer Sean Doolittle there by a year. He looked comfortable and social in the Nationals’ clubhouse over the weekend, and not just because he is so familiar with Chase Field.

The Nationals tried to get Reynolds to sign a minor league deal in spring training, but after he hit 30 home runs with Colorado last season, Reynolds couldn’t bring himself to come to camp having to make a team. Instead, he waited — until Nationals assistant general manager Bob Miller, who worked with Rizzo in Arizona, called.

They talked him into a deal, made sure it had an opt-out date in case he saw no path to the big leagues with the Nationals. Just when he had spent enough time at Class AAA Syracuse to start looking for other options, his old Virginia teammate Zimmerman hit the disabled list with a sore back.

“I took this offer because, number one, really good team. Number two . . . I thought if I didn’t at least try, I probably would never try again,” Reynolds said. “I didn’t want to be 45 and look back and say, dang, I could have played a couple more years.”

Instead, he is 34, looking as if he could emerge as a key piece for this much-injured Nationals team this summer, perhaps as a member of a platoon with Matt Adams — or, as happened Sunday night, joining Adams in the heart of the Nationals’ order until Zimmerman returns.

Questions about what happens to Reynolds when Zimmerman gets back are premature. Besides, with all their injuries, the Nationals will still have room for a right-handed power hitter on their bench somewhere. For now, Rizzo, Miller and the rest of the Nationals are just glad his path wound back their way, and that they will all fly into the Arizona night with a sweep of what had been one of the best teams in baseball, looking better than when they got here.