He cruised through the first four innings, allowing only three singles -- two of which were erased when the baserunners were caught stealing. Seven of the first 13 Washington outs came on strikeouts.
"Curveball was as good as it's been all year," Arrieta said. "I was able to throw it in the middle [of the plate], but underneath the zone, at will for the most part to give the appearance of a belt-high fastball."
In the sixth inning, Arrieta needed a Houdini act to escape with the lead.
"You've got to reset the mind every pitch," Arrieta said. "So that doesn't change there, mentally."
Harper notched his 45th RBI of 2015 -- and the first Nationals run in 18 innings -- but Arrieta won the battle, inducing a chopper to Starlin Castro at shortstop for the second out. Arrieta proved a bit wild once more by hitting Ryan Zimmerman, but when first baseman Anthony Rizzo was perfectly positioned to field an Ian Desmond line drive, the inning was over with the Cubs in front, 2-1.
Arrieta's day ended when manager Joe Maddon pinch-hit for him in the top of the seventh. He struck out eight on the night.
From there, the game was in the hands of the bullpen. Five relievers combined for three scoreless innings, making sure not to waste the gem that came before them.
Jacob Emert is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.