After a "Previously On" segment that makes it clear that this episode is 100% about the Gorn, we open with a Captain's Log from Captain Batel indicating that the USS Cayuga is visiting a colony world just outside Federation space, Parnassus Beta, which looks like a small 20th-century town, complete with a barber shop and a park with a jungle gym. Following her log, we see a short conversation between Batel and Nurse Chapel which indicates that Chapel was hitching a ride on the Cayuga to get to her fellowship. We then see a short call from Captain Pike to Batel with some cute banter which is cut short as comms go down.

We see a shuttle crash (we later learn this shuttle contained Scotty), and then the clouds open up to show a Gorn battleship.

Back on the Enterprise, they receive a distress call from Batel and the Enterprise immediately heads to the rescue. Pike calls Admiral April, who advises Pike to be cautious not to do anything that can start a war with the Gorn, reminding Pike that Parnassus Beta is not in Federation Space.

When they arrive, they are shocked to see the destroyed remains of the Cayuga. The Enterprise receives a message from the Gorn (relayed through Starfleet) indicating that the Gorn have indicated a demarcation line with Parnassus Beta on the Gorn side of the line and the Enterprise on the Federation side of the line. Starfleet orders the Enterprise to maintain the Starfleet side of the demarcation line.

There is some sort of interference field preventing the Enterprise from contacting anyone on the planet, or even detecting if there are any survivors there.

Pike decides to violate the orders and head to the planet, but they need to do so in a way that doesn't alert the Gorn that they've violated the demarcation line. Ortegas points out that the Gorn aren't shooting at the debris field from the Cayuga "because they already did." This gives them an idea to disguise a shuttle to look like a piece of debris, and then slowly drift through the field. Pike says he could use Ortegas' expert flying on this mission

<Ortegas> You want me to pilot the shuttle?
<Pike> Come on, Erica. Haven't you been bugging me for months about joining a landing party?
<Ortegas> Yes, I have. Why have I been doing that?

The mission works, though it requires the shuttle to free-fall through the atmosphere until it is under 1500 meters from the surface to avoid detection.

<Ortegas> I thought you were a test pilot.
<Pike> I was.
<Ortegas> Come on, it's like… it's like riding a bike.
<Pike> Yeah, at the speed of light! You were born for this, Erica.

On the planet, the damage is tremendous, and the crew looks for survivors, taking shelter from some juvenile gorn in the aforementioned Barber Shop. La'an points out that the Gorn are not acting normally; juvenile Gorn shouldn't be working together as a group, they should be fighting to assert dominance over each other (as they did on the previous Gorn episode from last season).

Continuing to look for survivors, they manage to fall into a "Gorn Trap" set by Lt JG Montgomery Scott, who was the one flying that shuttle that crashed earlier. His ship, and as far as he's aware, the entire crew of the Stardiver, is gone now. Scotty leads the team over to where Batel and the colonists are hiding.

Scotty also indicates that his crashed shuttle has a device he created that can somehow hide them from the Gorn. Unfortunately, this device is fused to the shuttle, so it will be hard to remove. Pike, Batel, and Scotty head to the shuttle to get the device.

While on the shuttle, working to get the device, they encounter a gorn that gets right into Batel's face, but then doesn't attack. Pike is, understandably concerned, and presses Batel to tell him the truth, she reveals that she was infected the day before with eggs, and has about a half a day left before the gorn burst out of her.

Meanwhile, back on the ship, Uhura and Pelia come up with a plan to disrupt the interference field by attaching some thrusters to the remains of the Cayuga saucer section to launch it down to the planet, crashing it into the massive tower that is the source of the interference field. Spock volunteers to take on the mission of attaching the thrusters.

He does an EVA to take the thrusters over to the seemingly derelict saucer section of the Cayuga without being detected by the Gorn, and when he gets there he meets up with Chapel who had survived the attack (seemingly the only crew member on the ship who survived), the two of them fight a Gorn on the broken bridge (wearing an EV suit which makes the gorn look... somewhat comical), and then eject through the dome of the bridge as the thrusters throw the saucer into the tower that creates the interference field.

The plan works, and the field drops. The Enterprise beams up Spock and Chapel, Chapel heads to Sickbay, and Spock to the bridge. They beam Pike, Batel, and Scotty to Sickbay so Batel can be sedated, and we see a transporter beam beam up the crew and colonists.

Pike is exceedingly happy to see Chapel alive, and Pelia gets reunited with Scotty.

<Pelia> Hello, Scotty.
<Scotty> Professor.
<Pike> You two know each other?
<Pelia> One of my best students... who sadly received some of my worst grades.

Scotty and Pelia take the remains of his device to engineering to figure out how to use it, and Pike heads to the Bridge, where it is revealed that the transporter beam that picked up Ortegas, George Kirk, M'Benga, and the colonists didn't come from the Enterprise, but in fact were picked up by the Gorn.

The Gorn attacks the Enterprise, and Starfleet sends the Enterprise orders to stand down and evacuate before the situation gets worse.

Pike looks on in horror while Una asks for orders, and we get the "To be continued..." card.