At the Amity Harbor, Deputy Hendricks jokes to Brody about Charlie's wife being upset about her roast being used as bait: "Mrs. Kintner must have put her ad in Field and Stream." Brody quips: "It's more like The National Enquirer." A boat arrives with a mangy-haired, fast-talking, hippie know-it-all marine biologist/ichthyologist (shark specialist) Matt Hooper (Richard Dreyfuss, after walk-on roles in The Graduate (1967) and The Valley of the Dolls (1967), and his appearances in George Lucas' American Graffiti (1973) and The Apprenticeship of Duddy Kravitz (1974)), one of the research scientist experts from the Oceanographic Institute requested by Brody. The rational fish scientist is offered a hand by Amity's local fisherman Ben Gardner (Craig Kingsbury) to help step up onto the dock from his boat.

Meanwhile, an armada of numerous boats prepares to leave the harbor dangerously overloaded with amateur shark hunters from all over New England. They are ready to haul bloody bait, beer, a bow and arrow, and even sticks of dynamite. When he sees the suicidal mission of "nuts" about to leave for the open ocean, Hooper turns away and chuckles to himself: "They're all gonna die." He also alerts an overwhelmed Brody about the eight guys in the overcrowded launch: "None of them are gonna get out of the harbor alive." Disgruntled about all of the out-of-town visitors, Gardner mumbles to himself about their stupidity as he heads out with them:

Wait'll we get them silly bastards down in that rock pile. There'll be some fun. They'll wish their fathers had never met their mothers when they start takin' their bottoms out and slammin' into them rocks, boy.

Hooper's first request of Brody is to see the remains of the first victim ("the girl on the beach") - Chrissie - allegedly a victim in the report of a "probable boating accident." In the morgue scene, Hooper thoroughly inspects the remains and describes the multiple injuries of the girl. [He refers to the shark as a squalus - the Latin word for shark. However, squalus technically refers to a family of dogfish sharks, not the great white sharks, which are classified in the lamnidae family.] He ends his exhaustive, gruesome examination with a 'plain-English' statement:

The torso has been severed in mid-thorax. There are no major organs remaining...The right arm has been severed above the elbow with massive tissue loss in the upper musculature...Partially denuded bone remaining. This was no boat accident. (To Brody) Did you notify the Coast Guard about this?...The left arm, head to shoulders, sternum and portions of the rib cage are intact...(He holds up the bluish, severed left arm/hand, with rings still on fingers, of the shark attack victim.) This is what happens. It indicates the non-frenzied feeding of a large squalus... Now, the enormous amount of tissue loss prevents any detailed analysis. However, the attacking squalus must be considerably larger than any normal squalus found in these waters....Well, this is not a boat accident, and it wasn't any propeller, it wasn't any coral reef, and it wasn't Jack the Ripper. It was a shark.

The camera pulls back from a closeup of the bloody, grinning, lop-sided mouth of a good-sized, real-life Tiger Shark as dozens of shark hunters display their recent trophy-catch on the dock. The shark looks like the victim of a lynching as it is hauled up by a rope for inspection. As the town's newspaper editor tells his assistant as they leave the scene: "I wanna go AP and UPI. I want to get on the state wire services, see if Boston will pick it up, and go national...," both Hooper and Brody approach from the right. Chief Brody smiles broadly at the bloody carcass: "(Did) Ben Gardner get this?!" Convinced and jubilant that his problems have ended, Brody happily poses for pictures with the triumphant fishermen, while a pensive Matt Hooper measures the fish's gaping mouth ("bite radius") with his tape measure. The majesterial Quint enters the harbor - derisively looking down on the scene from the upper bridge of his boat. As the mayor strides into the scene, assuring everyone that: "We can start breathing again," Hooper - in the foreground - backs away from other fishermen (off-screen) as he raises questions:

What I am saying is that it may not be the shark. It's just a slight - it's a difference in semantics that I don't want to get beaten up for.

Skeptical of everyone's conclusions, Matt Hooper takes the distracted Chief aside and tells him there is a distinct problem ("Martin, there are all kinds of sharks in the waters, you know. Hammerheads, white tips, blues, mankos, and the chances that these bozos got the exact shark...it's 100 to 1"). He is fairly certain that Chrissie was killed by a different shark:

Now I'm not saying that this is not the shark. It probably is, Martin, it probably is. It's a maneater, it's extremely rare for these waters. But the fact is that the bite radius on this animal is different than the wounds on the victim. I just...I want to be sure. You want to be sure. We all want to be sure, OK?

To confirm his conclusion, Hooper wants to cut open the slow-working digestive system of the shark and inspect it. The Mayor, who has been eavesdropping on their conversation, refuses with a plausible excuse:

Look fellas, let's be reasonable, huh? This is not the time or the place to perform some kind of a half-assed autopsy on a fish. And I am not going to stand here and see that thing cut open and see that little Kintner boy spill out all over the dock.

Obviously, the mayor plans to announce to tourists and newspeople that the crisis is over, and Amity Island is once again open to tourists - the beaches are now safe after the public menace has been eliminated.

A black-veiled, mourning Mrs. Kintner approaches, removes her face covering, and slaps Chief Brody hard across the face. The grieving mother is enraged and fighting to hold back tears. With her black veil swirling around her head in the wind, she believes her boy would still be alive if Brody had closed the beaches after the first shark attack:

I just found out that a girl got killed here last week, and you knew it. You knew there was a shark out there. You knew it was dangerous, but you let people go swimming anyway. You knew all those things, but still my boy is dead now. And there's nothing you can do about it. My boy is dead. I wanted you to know that.

Chief Brody stares at the ground, feeling miserable, knowing that the woman is correct in her assessment. The Mayor apologizes to Brody, believing she is wrong ("I'm sorry, Martin, she's wrong"), as Brody walks slowly away. He orders the tiger shark to be disposed of: "All right fellas, let's cut this ugly son-of-a-bitch down before it stinks up the whole island. Harv, you and Carl take it out tomorrow and dump it in the drink."

Later that evening, in a calming, quiet, believable homelife scene during dinner, a rather detached Brody sits with his son Sean at the table and contemplates the events of the day. His young son mimics his drinking gesture with his own glass of milk. The son also imitates the way his troubled father holds his hands in deep thought and then covers his face. When Brody finally notices that his son is copying his finger movements, he playfully makes funny faces at him (the son responds with clawed hands and a shark-eating face), and then lovingly asks for a kiss: "Come here. Give us a kiss...Because I need it."

Hooper visits the Brody's home (where the mood is somber), bringing both a bottle of red and white wine to their dinner table as a household gift. After Hooper helps himself to Brody's uneaten dinner plate, Brody's wife forces a conversation with him:

My husband tells me you're in sharks.

He describes his long fascination with sharks since boyhood ("I love sharks"), and he tells a tale from his past that actually foreshadows the conclusion of the film:

Yeah, I love them, I love them. When I was twelve years old, my father got me this boat and I went fishing off of Cape Cod. And I hooked a scup. And as I was reeling it in, I hooked a four and a half-foot baby thresher shark who proceeded to eat my boat. Ha, ha. He ate my oar, hooks, and uh, my seat cushions, he turned an inboard into an outboard. Scared me to death. And I swam back to shore. And when I was on the beach, I turned around and I actually saw my boat being taken apart. And ever since then, why yes, I have been studying sharks.

(During Hooper's long-winded discussion, Brody peels open and un-cork-screws the red wine bottle, and pours it into his water glass without letting it breathe.) Hooper plans to "go to the Institute tomorrow and tell them that you still have a shark problem here...They caught a shark, not the shark. Not the shark that killed Chrissie Watkins and probably not the shark that killed the little boy." Hooper learns from Brody's wife (when she playfully kids her husband and lovingly touches his arm) that he doesn't even like the water, swimming or boating:

Martin hates boats. Martin hates water. Martin - Martin sits in his car when we go on the ferry to the mainland. I guess it's a childhood thing. It's a - there's a clinical name for it, isn't there?
Martin (snapping back): Drowning!

Martin asks Hooper whether it is true that most people are attacked by sharks in three feet of water and about ten feet from the beach. Hooper confirms that he is correct. Brody also worries that the attacking shark won't go away if people keep swimming at the beach - providing fresh, human food for the shark: "Now this shark that, that swims alone...rogue, yeah, now this guy, he - he keeps swimming around in a place where the feeding is good until the food supply is gone, right?"

To get the opportunity they missed earlier in the afternoon, Brody and Hooper proceed down to the city pier and cut open the Tiger shark to examine its insides - the contents of its 'dinner plate.' (During the scene, a fog horn sounds in the distance.) They find only fish, a scavenged tin can, and a Louisiana license plate with the state's slogan: "Sportsmen's Paradise." With a straight face, Hooper claims that the shark came from the South: "he came up in the Gulf Stream, from southern waters." Brody matches his humor: "He didn't eat a car, did he?" Brody realizes the danger facing Amity - the shark has claimed Amity Island as his territory. And he knows that his fear of the water will weaken his efforts:

Brody: We've got to close the beach. Call the mayor.
Hooper: You've got a bigger problem than that, Martin. You've still got a hell of a fish out there with a mouth about this big (gesturing).
Brody: How do we confirm that by morning?
Hooper: If he is a rogue, and there's any truth to territoriality at all, we've got a good chance of spotting him between Cape Scott and South Beach.
Brody: Where are ya going?
Hooper: I'm gonna find him right now. He's a night feeder.
Brody: On the water?
Hooper: Well, if we're lookin' for a shark, we're not going to find him on the land.
Brody: Yeah, but I'm not drunk enough to go out on the boat.
Hooper: Yes, you are.
Brody: (disagreeing) No, I'm not.
Hooper: Yes, you are.
Brody: I can't do that.
Hooper: Yes, you can.

Out on Hooper's high-tech oceanographic boat on a night-time ride, they hunt for the shark on the theory that sharks are "night feeders." Brody drinks wine and opens up about why he has moved to Amity Island from the urban jungle of New York - to find a safe haven for his family:

I'm tellin' ya, the crime rate in New York will kill ya. There's so many problems, you never feel like you're accomplishing anything. Violence, rip-offs, muggings. The kids can't leave the house. You gotta walk 'em to school. But at Amity, one man can make a difference. In twenty-five years, there's never been a shooting or a murder in this town.

They cruise right to the stretch where the shark has been feeding. Admitting that he is part of a rich family, Hooper tells the Police Chief that he financed the boat's sophisticated closed-circuit TV system, underwater cameras and expensive fish-finder radar. Both men find themselves in ironic situations:

Brody: It doesn't make any sense when you pay a guy like you to watch sharks.
Hooper: Well, uh, it doesn't make much sense for a guy who hates the water to live on an island either.
Brody: It's only an island if you look at it from the water.
Hooper: That makes a lot of sense.

In their search for the shark, they come across the damaged, abandoned fishing boat of Ben Gardner and Brody asks: "What happened?" Hooper puts on his scuba gear to inspect the hull from underwater. Brody is uneasy and anxious, wishing to tow it in first, but Hooper insists ("I've just got to check something out"). Diving into the murky, claustrophobic water, Hooper finds a huge, serrated tooth embedded in a large hole in the hull. Then, in the biggest, single-most shocking moment of the film, when he examines the tooth and comes in for a closer look, the severed head of Gardner's corpse appears in the gaping hole. Hooper, scared to death (with the audience), accidentally drops the tooth as he struggles to the surface of the yellowish water.

The next day, Brody and Hooper talk heatedly to the mayor on the hillside where the Amity Island billboard is located. Brody is fearful and warns the Mayor about the conclusions of their findings the night before: "This is a Great White, Larry, a big one. And any shark expert in the world'll tell ya it's a killer. It's a man-eater." Hooper also explains the danger:

Hooper: Look, the situation is that apparently, a Great White Shark has staked a claim in the waters off Amity Island, and he's going to continue to feed here as long as there is food in the water.
Brody: And there's no limit to what he's gonna do. I mean, we've already had three incidents. Two people killed inside of a week, and it's gonna happen again. It happened before. The Jersey beach. (He points at Hooper.)...1916 - there were five people chewed up in the surf -
Hooper: - in one week.
Brody: Tell him about the swimmers.
Hooper: A shark is attracted to the exact kind of splashing and activity that occurs whenever human beings go in swimming. You cannot avoid it.
Brody: You open the beaches on the 4th of July, it's like ringing the dinner bell for Christ's sakes.
Hooper: Look, Mr. Vaughn. Mr. Vaughn, I pulled a tooth the size of a shot glass out of the wrecked hull of a boat out there, and it was the tooth of a Great White.
Brody: It was Ben Gardner's boat. It was all chewed up. I helped tow it in. You should have seen it.

Mayor Vaughn logically argues back, smirking to himself that they don't have proof of the tooth:

Where, where is that tooth? Did you see it, Brody?...(To Hooper) And what did you say the name of this shark is?...You don't have the tooth.

The stubborn mayor still maintains an attitude of business first:

Mayor: Look, we depend on the summer people here for our very lives.
Hooper: You are not going to have a summer unless you deal with this problem.
Mayor: And if you close those beaches, we're finished. (Overlapping with Hooper)
Brody: We're not only gonna have to close the beach, we're gonna have to hire somebody to kill the shark...
Mayor: I don't think either one of you are familiar with our problems.
Hooper: I think that I am familiar with the fact that you are going to ignore this particular problem until it swims up and bites you on the ass...There are two ways to deal with this problem: you're either going to kill this animal or you're gonna cut off its food supply.
Brody: Larry, we have to close the beaches!

The mayor responds to the "sick vandalism" by ordering Brody to apprehend the graffiti vandals who have defaced the public service billboard ("a deliberate mutilation of a public service message"), painting in a dialogue balloon with the words "Help!! SHARK" coming from the mouth of the swimmer on a surfboard:

Now I want those little paint-happy bastards caught and hung up by their Buster Browns!

Hooper expresses disdain for the mayor who refuses to acknowledge that there is a shark problem:

I'm not gonna waste my time arguing with a man who's lining up to be a hot lunch...Mr. Vaughn, what we are dealing with here is a perfect engine - uh, an eating machine. It's really a miracle of evolution. All this machine does is swim and eat and make little sharks, and that's all. Now why don't you take a long, close look at this sign. Those proportions are correct.