As the test approaches, the plutonium bomb project keeps getting hit with one crisis after another.
Kistiakowsky realizes that his custom-made explosives are full of air holes (which, when perfection is needed, are unacceptable).
Oppie begs Groves to allow him to delay the test, but Groves denies him: Truman is on his way to Potsdam to meet with Stalin to discuss post-war plans, and the president needs to know for a fact (as an intimidation strategy) that the U.S. has a working atomic bomb.
Once the scientists realize there isn't any flexibility, they find ways around the various crises. Kistiakowsky ends up using a dental drill to access the air pockets in his highly sensitive explosives, and he fills the holes with liquid explosives, drop by drop.
Meanwhile in southeast England, the British have made a little slice of Heaven for their ten captured German scientists at a place called Farm Hall. The Germans will live in the lap of luxury, and can do whatever their hearts desire—except leave.
This plan is twofold. One: they want the scientists kept in complete isolation so they don't spread the word about the existence of an atomic bomb, and two: the whole place is wired with tiny microphones so the British can learn how much the Germans have actually figured out about building atomic bombs.
Back in Los Alamos, the test is a go. Philip Morrison, one of Oppie's former students, has the privilege of driving the plutonium bomb core out to the Trinity site. He's struck by how odd it is to just casually drive with the worst bomb in human history next to you in the back seat.
Next, Kisty's five-foot-wide ball of explosives is carefully driven out to the test site.
The scientists start to assemble the bomb at the site; Oppenheimer acts like a nervous father-to-be whose wife was in labor the whole time.
Major Robert Furman and Captain James Nolan accompany the shipment of U-235 from Los Alamos to the Pacific island of Tinian, where pilot Paul Tibbets is waiting with his crew. Regardless of how the plutonium bomb test goes, the uranium bomb is to be assembled and ready for use by early August.
Back at Trinity, the bomb is assembled and raised onto its wooden platform tower, where the scientists start attaching wires and detonators all over it.
Now they just have to wait until 4:00AM, which is the time decided for the test. Until then, chemist Donald Hornig gets to babysit the bomb, which seems like an anxiety-inducing activity.
Unfortunately, despite weeks of clear weather, a huge thunderstorm arises over the site, which has Oppie really worried. Not only will visibility be poor, but also setting the bomb off in a storm could mean that the rain clouds carry the fallout over nearby towns, spreading the radiation. That would be bad.
Groves and Oppenheimer settle down to wait out the storm.