Lee looks in the sink, where we can see tendrils snaking their way towards the drain. Walter figures they're seeking more water and nutrients, showing a strong impulse to expand their reach as far as possible. It's one of those instances where Walter seems delighted by whatever creepy new killer they've discovered. Broyles wants to know, "admiration aside, Dr. Bishop," how they kill the stuff. Lemon Pine-Sol? Nope, strong dose of UV light or intense heat should do the trick.
So Broyles orders an "Agent Mace" to get as many high-intensity U.V. lamps they can find, and he tells Olivia to get a team together with Lee and meet at the tunnel. Olivia says, though, that the fungus colony at the tunnel is probably much bigger: "I'm not convinced that UV is the most efficient way to destroy it." She's thinking flamethrowers. I'm with Olivia! I have always wanted a flamethrower. It would be useful around the house! Plus a car-mounted one would, I hope, stop me from getting cut off in traffic.
Back at the Harvard lab, Walter tells Aaron that he shows no sign of fungal infection, which means he's free to go home. Astrid will take him. Walter notices how decidedly non-thrilled the boy is by this news. "Nobody's there," says the kid, who adds that the neighbor he stays with never talks to him. Well, so much for the Mrs. Robinson fantasy I always had about staying with the neighbors when I was this kid's age.
There's no place else for him to stay, he says, and Walter sympathizes: "I certainly know what it feels like to have nowhere to go, no family. This is the only home I know," he says, gesturing with his head around the room.
Aaron looks around. I can't tell if he's impressed or alarmed by the news that Walter lives here. "It's not much, I admit, but ... it's certainly better than the mental institution where I lived before this," he says. The kid says nothing, and then slowly starts to walk away, until Walter asks if he'd like to stay here a little while longer. Aaron brightens considerably. Walter sternly says he can't promise he'll have any time for entertaining him, being so busy.
Cut immediately to the two of them listening to music while wearing tinfoil hats (stylish and practical!) and drinking strawberry milkshakes. Aaron's so thrilled at having someone to spend time with that he figures why not bring the mood down a little by asking how Walter's son died.
Walter explains the backstory as we know it right up until the part where Walter brings the other Peter back from the other universe. "But the frozen lake where I created a portal between universes was unsound, and when we crossed back, the ice broke. And Peter -- the other version of him -- drowned. And I lost him all over again," he says. The music has very respectfully turned itself off for Walter's speech.