After the homecoming dance, in the tree outside Lana's bedroom window, Greg Arkin, an acne-ridden, insect-loving teen, secretly videotapes Lana. At home, Greg finds his mother, Arleen, watching one of his old videos of Lana. She's angry and disgusted at both of Greg's habits, his stalking Lana and his insect collecting. Greg decides to run away, but as he drives out of town, some of his insect cages break open, Greg crashes into a telephone pole, and the insects swarm around him, stinging and biting. Later that night, we see Greg in his room, covered with bug bites, clinging insect-like to the ceiling.
Clark has a dream of flying and awakens to find he is floating. He then crashes down, crushing his bed beneath him. Later, Clark is helping his parents at the Smallville Farmers Market. Whitney and Lana arrive and stepping away from Lana, Whitney tells Clark he needs Lana's necklace back. Clark doesn't have it. At the same time, Greg, looking and feeling supremely confident, makes a date with Lana for her to help him with some homework. Whitney tells Greg to stay away from Lana, but Greg isn't daunted. Later, as Whitney is driving home, Greg leaps from a tree onto Whitney's truck causing Whitney to crash. The Kents drive up and see Whitney unconscious inside. Clark drags Whitney out, then shields him with his body as the truck explodes in a huge ball of flame. Horrified, Jonathan and Martha rush to Clark, amazed, but relieved, to find that he is alright. Back at home, Clark tells Jonathan about that morning's floating incident; "What's happening to me?" he asks. Jonathan doesn't know but assures Clark they'll work through it. Meanwhile, Lex suggests to Lana that she's dating the wrong guy, and cryptically tells her she should ask Whitney what he was doing before the homecoming game. Back at Greg's house, Arleen arrives home to find the thermostat set at 103 degrees, and Greg's room covered spider silk. Greg appears behind her, suggesting that he is now the end product of millions of years of insect evolution. First he'll eat, then he'll molt, and then he'll mate. Terrified, Arleen tries to back away, but Greg stops her with a torpedo of wet silk he shoots from his mouth.
At Luthor Mansion, Lex gives Clark the "Trojan horse" to Lana's heart: her necklace. Lex opens the box he's keeping it in, and Clark is overcome with weakness. When Lex closes the box, Clark feels fine. Intrigued, Clark asks Lex what the box is made of. "Lead." At school the next day, Lana confronts Whitney about making Clark the "Scarecrow," and asks for her necklace back. When Whitney admits he lost it, Lana is furious and storms off, looking for Clark. Greg bumps into her, and reminds her of their study date. Lana says she can't talk, she has to see Clark. Greg is angry and jealous. Back at the Kent farm, Clark tests the effects of Lana's necklace. When he opens the lead box he immediately feels weak and sees that his veins are black, pulsing and throbbing beneath his skin. When he closes the box he feels fine. Unsure what to make of it all, Clark returns to his loft to find Lana waiting for him. He quickly hides the box. Lana apologizes for Whitney's behavior. She thought she knew Whitney and now wonders what else she's been blind to. She tells Clark that Whitney even lost her favorite necklace - it was made from a fragment of the meteor that killed her parents. Meanwhile, Greg is in the shower, shedding the outer layer of his skin as he molts. That night, Greg attacks Clark in the barn. Greg then pushes Jonathan from the barn rafters, hurling him towards the upturned tractor blades below. As Jonathan plummets in slow motion, Clark speeds under Jonathan, breaking his fall and saving his life.
In the aftermath of Greg's attack, Clark confesses to Jonathan that he feels responsible for the bad things that happen in Smallville since they seem to be caused by the meteors that accompanied Clark's arrival on Earth. At school the next day, Clark tells Chloe about Greg. After some research, Chloe theorizes that Greg has transformed into some sort of "bug boy." Realizing Greg is now in the mating phase, Clark races to Lana's house. Meanwhile, Whitney is with Lana apologizing for how he treated Clark. Suddenly, Greg arrives. With superhuman strength, Greg hurls Whitney aside and tells Lana "It's time... for us."
Whitney is recovering from Greg's attack when Clark arrives. Clark tells Whitney to head for a tree fort that Clark and Greg used to play in as kids. Clark then runs ahead to the tree fort, finding Lana cocooned in silk, and Greg lying in wait. Greg pushes Clark through the wall of the tree fort, and they both fall to the ground. Greg then jumps a fence and heads for an abandoned foundry. Clark follows. Unfortunately, the foundry is filled with meteors and Clark is weakened the moment he steps inside. Greg attacks Clark, getting the best of him until Clark makes his way into a metal vat made of lead. His strength renewed, Clark fights back. Greg accidentally pulls a lever that sends a massive weight crashing down on him. Meanwhile, Whitney has found the tree fort and rescues Lana. Clark arrives just in time to see the two of them together, walking arm in arm back to Whitney's truck. That night, Clark anonymously leaves Lana's necklace hanging on her front door. Lana is thrilled and grateful to have it back.