When the Nationals’ most needed a run, they turned to Harper. Johnson had held him out because coaches and trainers alerted him to stiffness in Harper’s lower back. Harper protested, but still sat. All game, he insisted he could play, and Johnson waited until the right moment.

“I’m pretty antsy when it comes to sitting,” Harper said. “I don’t like doing it. I like to get going and play.”

He pinch-hit for Tyler Moore in the ninth, with one out and the bases empty. When he saw catcher Jarrod Saltalamacchia meet Red Sox closer Alfredo Aceves on the mound, Harper thought, “He’s probably not going to give me anything.”

Aceves started him with three straight curveballs, only one of them of a strike, and that one barely over the inside corner. Harper spit on a low fastball and then took another fastball for ball four.

Up came Bernadina. He took a close ball, just off the inside corner, on 1-2. (Boston Manager Bobby Valentine would be ejected arguing balls and strikes on the game’s third-to-last pitch. “It was a ball,” Bernadina said.) On 2-2, Harper took off, a straight steal. Bernadina crushed it into right field.

“I was looking for his fastball,” Bernadina said. “He threw me one up, and I reacted.”

As the ball zipped past him, Harper thought, “I’m scoring.” The ball skipped into the corner. Third base coach Bo Porter anticipated sending Harper as soon as the ball left Bernadina’s bat. Right fielder Ryan Sweeney cut off the ball, but he had to leave his feet and slide to do so.

“That’s when I made up my mind,” Porter said. “I’m going to send him.”

Harper never hesitated, and his speed took over — Nationals coaches regularly time him running to first base in 3.96 seconds. As he rounded second, he locked his eyes on Porter, who windmilled him home. As Harper rounded third, his helmet nearly flew off his head. Adrian Gonzalez gathered Sweeney’s throw and fired it home. Harper slid in feet first, tapping the plate with his left hand. He hopped to his feet and pumped his fist.

For the run to matter, Espinosa had to land his pivotal blow. In the seventh, consecutive singles and a double steal by Ian Desmond and Moore put runners on second and third with no outs. But Jesus Flores whiffed and Bernadina popped up a bunt. The inning, packed with promise, fell to Espinosa.

Lester threw Espinosa a 2-1 cutter, and he launched the ball high to left field. Darnell McDonald drifted back, closer and closer to the Monster. Espinosa thought, “Ah, I didn’t get quite enough of it.” Desmond and Moore raced around the bases, certain runs depending on what happened to the ball falling out of the sky.

McDonald leapt. The ball thudded and rolled toward the infield. At 29 other ballparks, Espinosa’s drive would have been a deep but routine flyout. At Fenway, he rolled into second base with a double and the Nationals took a 3-2 lead.

“I’m pretty happy about that wall,” Espinosa said.

Zimmermann gave up the lead in the bottom of the seventh, but that only set up Bernadina’s laser and Harper’s dash. From the dugout, Johnson watched his young team embrace Harper as he walked into the dugout.

“It was picture perfect,” Johnson said. “Storybook perfect.”