Hopper manages to evade the lab's security and discovers a portal in the basement of the lab. But no sooner has he found it, before the lab's guards catches him and knock him out. He wakes at home and discovers a hidden microphone. His deputies pay him a visit to inform him that Barb's car was found, but Hopper finds the state's involvement suspicious.
The boys determine Will is trapped in an alternate dimension Eleven calls the "Upside Down," and he is being pursued by a monster similar to a Demogorgon. After Will's funeral, the boys ask their science teacher Mr. Clarke about alternative dimensions. He explains that a high-energy spacetime tear could theoretically create a passage between dimensions, and that such a phenomenon would create a disruption in the Earth's electromagnetic field. Using their compasses, the party heads into the woods to search for such a disruption. Eleven remembers encountering the creature during a lab experiment, and afraid, she redirects the compasses. Lucas notices and confronts her. Mike defends her, he and Lucas fight, and Eleven telekinetically flings Lucas off Mike. While Dustin and Mike tend to the unconscious Lucas, Eleven disappears.
Lonnie comes back to town ostensibly to console Joyce about Will's disappearance. The two make peace with each other, until Joyce discovers that Lonnie still doesn't actually care about his sons, but is really just trying to cash in on Will's death with a wrongful death lawsuit. Angry, she demands that he leaves again.
In the woods, Nancy and Jonathan find a wounded deer. The creature drags the deer away, and Nancy follows the trail of blood to a portal to the Upside Down. She crawls through and discovers the creature feasting upon the deer. She snaps a twig and draws the creature's attention.
This episode provides examples of:
- All Just a Dream: Played with; after finding Hopper snooping around their lab, Hawkins' employees drug him and bring him back to his house, placing pills and empty cans around him, in hopes that Hopper will believe everything he saw there was just a bad dream.
- Armor-Piercing Question: When Joyce discovers Lonnie's plan to cash in on Will's death with a wrongful death lawsuit, his justifications seem entirely plausible and consistent with being a caring father: paying for Jonathan's college, for instance. However, Joyce lays bare the speciousness of his explanations by asking the name of Jonathan's dream school — which Lonnie cannot answer.
- Awful Wedded Life: Jonathan and Nancy discuss their parents' marriages as this,
- Batter Up!: Nancy picks a baseball bat as her weapon for monster-hunting.
- Fold the Page, Fold the Space: In Episode 5, the science teacher uses this method to explain to our kid heroes how they could create a doorway to the "Upside Down" dimension. He takes a paper plate, folds it and pierces it with a pen.
- A not-entirely-deliberate example (or, at least, one that touches on Entertainingly Wrong), since Lonnie doesn't actually know about the genuine paranormal events occurring within the house and genuinely does think Joyce is simply having a breakdown out of stress and grief. However, he's barely been back a day before managing to almost convince Joyce that she actually is losing her mind and just imagining seeing Will again, and he brings up the idea of suing the quarry company in a rather insidious fashion that smacks of this kind of manipulation. She quickly snaps out of it upon discovering the compensation forms he's planning to use to try and cash in on Will's death, however, and it's implied that this isn't the first time Joyce has had to deal with Lonnie playing these kind of mind games with her.
- The Hawkins Lab people also try this on Hopper to make him believe that his investigation of the lab and discovery of the portal to the Upside Down was simply the result of an hallucination during a drug bender. He doesn't buy it, however, and instead tears his place apart until he discovers the bug they've planted to monitor him.
- Get Out!: Joyce yells this phrase at least three times at Lonnie after she finds out he's only there to cash in on Will's death.
- How Many Fingers?: Dustin does this after he and Mike revive an unconcious Lucas.
- Jerkass Has a Point:
- Sure, Lonnie is a deadbeat dad trying to cash in on his son's apparent death, but he is right: that gorge is ridiculously unsafe. It's just a sheer dead drop over the side into the water around a hundred feet down. Later, we see Mike nearly fall in, too. Even if Will really wasn't killed there, someone likely would be.
- The boys are hurt, betrayed and angry when it's revealed that Eleven has been manipulating their compasses to direct them away from the source of the disruption. Of course, they don't know but the audience does both what kind of horrors Eleven has experienced there and the dangers currently occuring at that particular location, making Eleven's actions entirely understandable (if not exactly admirable).
- Love Makes You Dumb: In-universe, Lucas views Mike's feelings for Eleven this way.
- Match Cut: Someone hammering a nail is used to segue into Mike poking a hole into a paper to illustrate the titular "flea/acrobat" metaphor.
- Meaningful Funeral: Granted, it might be a bit more meaningful if we didn't strongly suspect at this point that Will wasn't actually dead, but nevertheless the funeral for an apparently deceased eleven-year-old is treated with all the solemnity such a situation would require. As is underlined by the choice of music, New Order's "Elegia".
- Mercy Kill: Jonathan and Nancy find a deer in the woods that has been hit by a car. As they decide what to do and screw up the courage to shoot the poor thing, the Demogorgon makes the decision for them and drags it off to feed.
- Perspective Reversal: Up to this episode, Jonathan has been convinced that his brother's disappearance has been under natural circumstances and that he's now dead, while Joyce has insisted that something unusual is happening and that he is still alive. In this episode, thanks to discoveries made in the previous one Jonathan has now come to believe that his mother is right, while Joyce (thanks in part to Lonnie's manipulations) begins to doubt herself and believe she's going insane. Ultimately subverted, however, since Joyce wises up to what Lonnie is doing and realises that she isn't going crazy after all.
- Pet the Dog: Subverted. Lonnie seemingly comes back to make things right and, though he doesn't believe Joyce, treats her with care. Then The Reveal about his true intentions occurs.
- Plot-Mandated Friendship Failure: Lucas and Eleven have a fall out which makes the former decide to go and look for Will all by himself and the latter to leave the group until her Big Damn Hero return at the end of Episode 6.
- Properly Paranoid: Hopper not only doesn't fall for the trick to make him believe his trip to Hawkins lab was just a dream, he also correctly predicts that Hawkins' people might be keeping an eye on him now that he's onto them and thoroughly searches his house for any microphone, which he eventually finds. Later, when visiting Joyce, he urges her to stay quiet until he has searched her house for any microphone, as well.
- Reality-Breaking Paradox: The gate. Mr. Clarke points out that though science recognizes that there could be ways to alter the laws of physics, doing so anywhere near anything anyone cares about would be... irresponsible."Well, if it did, I I think wed know. It would disrupt gravity, the magnetic field, our environment. Heck, it might even swallow us whole. Science is neat... But I'm afraid it's not very forgiving."
- "The Reason You Suck" Speech: Jonathan delivers one to Nancy during their argument in the woods.Jonathan Byers: Well, I was just starting to think you were okay. I was thinking, 'Nancy Wheeler. She's not just another suburban girl who thinks she's rebelling by doing exactly what every other suburban girl does, until that phase passes and they marry some boring one-time jock who now works sales, and they live out a perfectly boring little life at the end of a cul-de-sac just like their parents, who they thought were so depressing, but now hey, they get it.'
- The Runaway: Barb's disappearance is covered up by making her look like this.
- This Is Gonna Suck: Hop storms into Joyce's house with a sign warning her to keep quiet, it might be bugged. He starts scoping how he'll start his search. . . and sees all the strands of Christmas lights. His expression is 100% this trope (lights being excellent places to hide bugs, easy access to electricity for them). When we catch back up to them, Hopper is sitting with an exhausted posture and expression that says "Yeah, that really did suck."
- Title Drop: The episode title comes from Mr. Clarke's metaphor about dimensional travel.
- Weirder Than Usual: Verbatim, when Lucas asks Dustin if he thinks El is acting weird.