"That tunnel goes under the river. There's hundreds of people in it at any one point," says Olivia, and just in case we don't get it, she adds, "If that tunnel ceases to exist, they'll all drown." Broyles orders Lincoln to the scene to co-ordinate an evacuation with Boston PD.
And then Astrid comes up with the "Walter Bishop Faraday Harness" that he threw together and she schlepped down there in all of about five minutes, and Peter and Olivia briefly spar over who gets to wear it, with Peter appealing to Broyles and winning because they need someone with a science background, since they don't know if whoever goes in is even going to be able to communicate with Walter once they get in.
Walter gives instructions to Astrid via his video-link on how to make sure the Faraday Harness is put on right. "The primary transistor goes on the back of the neck with the prongs either side of the wearers spine," says Walter, which is around the point that Olivia's probably glad she's not the one going in.
Astrid shows Peter the giant plug. "This goes into your neck," she says. "Course it does," says a resigned Peter. And given that the prongs are supposed to go on either side of Peter's spine, Astrid jams it in a little too quickly for my comfort level.
Anyway, Walter figures this will get Peter in, but he can't guarantee what happens on the inside. Astrid asks what happens if it breaks down on the inside. "Exactly the same as happened to the unfortunate fellow who turned into confetti," says Walter. Well, that's reassuring! Walter looks like he's thinking, "Ehh, either way."
Peter slowly makes his way up the path, everyone watching anxiously. He turns on the harness, flexes his fingers and then extends his right hand towards the invisible sphere. He puts his right hand in, he puts his right hand out, he puts his right hand in and he shakes it all about. Then he closes his eyes and steps through. Everything seems copacetic so far, and Astrid tells Walter that Peter's in. And despite his harshness all episode, Walter clearly looks relived. "The cage was a good idea. He's very smart," he says.
Down in the basement, Raymond is urging Kate to keep working, but she says it's over, the police are outside. Raymond tells them it's impossible for them to come in (but he didn't factor in the big brains of the Bishop boys!) and she just needs to solve the problem.
Looking practically near tears, Kate says she already solved it; that's what she was trying to tell him upstairs. "I haven't written it down yet, but I have it."