The Boathouse Fire Body Heat (1981)

The film's climax is a single delayed-trigger detonation

Body Heat's climax is staged in three close beats — the kitchen scene with Edmund's gun on the counter, Matty's walk down the lawn to the boathouse, and the explosion that engulfs her at the door.^{b37 b38 b39} The whole sequence runs about six minutes. The detonation itself is one shot: Matty reaches the door, the boathouse engulfs in a single fireball, and Ned in the trees screams her name. The cut is direct from the explosion to the interview room: "She's alive!"^b40

The kitchen scene is built on Ned finally voicing the suspicion

What gives the climax its specific weight is the kitchen scene that precedes it. Ned has driven to the Pinehaven house, set Edmund's gun on the counter, and waited.^b36 Matty arrives back from Miami, sees the gun, and asks what it's for. The dialogue Lawrence Kasdan (in Body Heat) gives them is among the film's most quoted:

"Keep talking, Matty. Experience shows I can be convinced of anything." — Ned Racine (William Hurt), Body Heat (1981)

"No matter what you think… I do love you." — Matty Walker (Kathleen Turner), Body Heat (1981)

The line is the climax's hinge — said by a woman walking knowingly into the trap she set. Turner plays the line straight; she does not telegraph the con. The audience, on first viewing, cannot tell whether she is lying or telling the truth, which is the whole technical achievement of the performance.

"Turner gets to deliver one of the great climactic lines in the noir tradition — and she has to deliver it as if it were both true and false simultaneously, because the film's design depends on the audience not being sure. That she pulls it off in a debut performance is the central marvel of Body Heat." — Roger Ebert, RogerEbert.com (Great Movies, 2002)

The walk down the lawn is the longest near-silent stretch in the film's back half

After the kitchen, Matty walks out the kitchen door and down the lawn toward the boathouse. The shot is nearly silent — no dialogue, John Barry's score reduced to a single sustained string note, and the sound of cicadas. Richard Kline's camera holds the wide shot from inside the kitchen as Matty's white skirt recedes into the dark yard. Ned stands at the kitchen window. He does not move.

The patience of the staging is what makes the detonation land. Kasdan trusts the audience to know what is about to happen. The walk is long enough that any viewer who has been paying attention has had time to put it together — Teddy's "this broad came to me last week," the door delay, the second device — and to realize, before the explosion, that the trap is rigged for whoever opens the boathouse.

The detonation is a single shot

The boathouse explodes as Matty reaches the door. Special-effects coordinator John Frazier supervised a real-fire detonation on the Lake Worth, Florida set; the boathouse was a constructed practical built specifically for the shot. Kline shot the explosion with three cameras — a wide from the kitchen window, a long lens from a position behind the boathouse looking back at the house, and a high-angle drone-style platform that captured the fireball from above. The wide is the shot used in the cut.¹

The single-fireball staging is unusual for a 1981 thriller, which would conventionally have built the climax through a montage of multiple cuts and angles. Kasdan let the explosion be one shot.

"There is one explosion. One shot. That's it. The temptation is always to cut six times and milk it. Larry refused. The single shot is the reason the climax is so devastating — you have time to register that this woman is gone and Ned is still standing there." — Lawrence Kasdan, The Hollywood Reporter (2021)

The body in the boathouse is not Matty

The cut to the interview room reverses the climax: "She's alive!" Ned proposes that the body in the boathouse was Mary Ann Simpson's, planted there waiting.^b40 Oscar replies that the body was identified by dental records sent back to Illinois — positive ID, that was Matty Tyler Walker. Ned starts to invert it: maybe the woman he knew as Matty has been using a dead Matty Tyler's name since she met Walker three years ago.

The structural fact the climax stages — and the prison-library reconstruction confirms — is that the boathouse explosion is Matty's climax, not Ned's. From her POV the entire film is the execution of a long con whose climax is the disposal of Mary Ann (the only person who knew her real identity) and Ned (the only person who could later supply a corroborating witness against her) in one delayed-trigger detonation. See Plot Structure (Body Heat) for the doubled-quadrant reading.

The explosion is the rhyme to the porch break-in

Body Heat is engineered so that the porch break-in (the inciting incident) and the boathouse fire (the climax) are the same image: a glass door, a person on the threshold, a violent thing on the other side. In the inciting incident Ned breaks the glass to get in. In the climax Matty walks through the glass and the violent thing is on her side. The film's structural argument — that Ned has been the instrument from frame one — is delivered as a closed rhyme. See The Window-Smashing Entrance.