A teenage boy rides through the desert on an off-road bike. He stops when he spots a tarantula. He reaches into his jacket for a jar he clearly had prepared for the purpose and carefully places the spider into the jar and leaves on his bike.
Walt stops by Hank's office and asks Gomez for a few minutes alone with Hank. Hank makes mention of Walt's new expensive watch. They talk about Skyler and how the kids are doing at Hank and Marie's place. Eventually Walt blurts out that Skyler doesn't love him any more. He starts to cry and Hank becomes incredibly uncomfortable. So much so that he leaves the office to get them coffees. Once that happens Walt springs into action, placing some kind of device on Hank's computer and planting a bug in one of the photo frames on his desk.
Walt, Jesse and Mike bring Lydia to some kind of remote underground location. Mike tells her that she'd be dead if it were up to him, but her last chance involves calling Hank and reading from a prepared script. If she does anything they don't like Mike will shoot her in the head. She calls Hank and tells him about the tracking device she found on the canister. He tells her to put the drum aside and do nothing. After the call, they listen to Hank ask Gomez about the tracking device. Clearly it didn't come from that office At this point they are set to kill her, though Jesse still doesn't like it. Just as Walt explains to Jesse the vote is 2-to-1 in favor of killing Lydia, they hear Hank call the Houston office. An agent there says his office was responsible for the tracking device. Mike still wants to kill her, telling the guys about her putting a hit out on him. Lydia doesn't deny the accusation but tells them she knows where they can find an "ocean" of methylamine.
Lydia tells Walt there is a regular train delivery of 24,000 gallons of methylamine that makes a trip from California to places like Texas and Oklahoma where the chemical will be used in the manufacture of pesticides. There is a three-mile "dead zone" in the trip where there is no access to cell phones. Mike thinks there is no way they can rob the train without killing the two men who operate the train.
We see that Walter Jr. is miserable with Hank and Marie, hardly ever leaving his room. They show alot of affection towards Holly.
Mike wants them to start making smaller batches using cold pills. He says not cooking is not an option and this leads to an argument between Mike and Walt about the nine men he's paying to keep quiet. Jesse wonders "What if we can rip off that train and nobody ever knows it got robbed."
The guys go to the train tracks and measure a stretch. They find an open area, an arroyo beneath a small bridge and Jesse says "It's perfect." With help from a backhoe, the guys bury two enormous plastic containers underneath the ground next to that section of track. Todd comes by and helps fill one with water. Walt and Jesse explain to him the plan is to fill one drum with 1,000 gallons of methylamine, then replace the missing chemical with water. When the train is weighed in a few days there will be no difference. And when the product is eventually used, all that will happen is that the buyers will think China sent them a slightly weaker batch.
Walter Jr. is locked in his room at home. Walt tells him he has no choice but to go to Hank and Marie's. After he's gone Skyler and Walt have yet another argument about the kids being in danger. She says she'll be whatever kind of partner he needs as long as the kids don't live with them at home.
Lydia calls Mike and tells him which tanker will be holding the methylamine. Kuby then drives a large dump truck onto the tracks and opens up the hood. He flags down the train, which manages to stop just a few feet before hitting the truck. Kuby says he's having engine trouble.
As soon as the train stops, Jesse and Todd spring into action. Todd climbs atop the tanker, removes bolts and inserts the water hose. Simultaneously Jesse gets underneath and connects a hose to the bottom.
Kuby continues to stall, flipping a switch of some kind that renders the truck inoperable.
After 300 gallons of methylamine are removed, Walt starts a generator and the water is pumped into the top.
With 600 gallons out a man in a truck approaches unexpectedly. He offers to push Kuby's truck off the tracks and there really isn't much he can do to stall. Kuby tries to prevaricate but is soon called on it by the train crew. Mike tells Walt to stop the operation, but with only 850 gallons of water replaced Walt seems to want to continue. Walt waits until all the measured water has been pumped, by which point the train is about to get under way. Todd and Jesse are able to finish up in the nick of time, with Todd having to jump off the moving train and Jesse having to lie underneath the train as it moves over the top of him. But the plan worked. As the guys laugh and celebrate, Walt looks over and sees the spider-hunting kid watching them from his bike. He gives them a tentative wave. Todd waves back and approaches the youth. Just then Todd reaches into his belt and pulls out a gun. Jesse screams "No!" but Todd shoots the accidental intruder.