It starts with a lengthy teaser--nearly five minutes long. There's a gorgeous close-up shot of the Seaview, quietly making her making her way home after a routine run, when they suddenly spot another submarine in the vicinity, which quickly opens fire on them. Battle stations are called, and they manage to avoid the first salvo of torpedos. They also avoid the second salvo by the length of a gnat's whisker--the torpedos pass just underneath. Crane concludes that it's a drone sub, which means that it's locked in on them. It is also equipped with a gadget for neutralizing Seaview's homing torpedos. Crane is forced to use a new weapon--an ultrasonic--which apparently is so new and dangerous that it's not supposed to be used without authorization. Crane uses it anyway--and that's the last we hear about it for this episode. They're all left wondering who attacked them and why.
The Seaview is preparing to run tests of a new automated system. (Wonder if it's related to the system used in "The Human Computer" the first season). The crew is to be stripped down to Nelson and Crane, and eight civilian scientists (actually, one is military) are coming aboard.
The unknown enemy (who operate out of a big fancy manor house) have learned about the tests. Their primary purpose at the moment is to destroy the "Peace Ship"--a ship with delegates from at least 7 countries on board to work out a peace treaty. Having failed to destroy Seaview, which presumably would be ready to defend the Peace ship if necessary, they decide to make Seaview herself their weapon, using one of the scientists coming aboard.
The enemy's agenda gets underway nearly as quickly as the Seaview's test--they get just far enough out to be completely cut off from aid before the power is cut off. The first scientist is killed off--Rourke, crushed in a turbine in the engine room. It's difficult to see just how the killer pulled this off, as nearly everyone was in the Control Room at the time. It's believed at first that Rourke's death was simply a tragic accident, but when Klaus is gunned down in the Control Room, it becomes obvious what is going on.
After distinctly warning the remaining scientists to remain in the Control Room, Nelson and Crane begin a search of the ship. (Interestingly, while accusations fly about like richoceting bullets, no one ever thinks to accuse Nelson or Crane. We know they're innocent, but the scientists certainly didn't!) Nelson and Crane are briefly trapped in a small room somewhere down around the engines, but manage to blow their way out using material from an emergency flare.
They return to the Control Room...to find only the two women remaining. The four remaining men have all wandered off somewhere. Nelson and Ava head down to her cabin, which is the only room that has not been searched, as it was locked. (You'd think Nelson would have a master key.) While there, Stroller (the military one) shows up and mentions that Nelson's lab is unlocked. Nelson rushes to check, and they find Carter head first in one of the aquariums.
Naturally, by this point, everyone's getting a little antsy. Perhaps that's the explanation for Nelson's odd accusation against Tracy. He acts as though she had been alone in the Control Room when he and Crane got there, although Ava had been there as well. Chandler seems more frantic than anyone. Despite being told that a mini-sub would be destroyed at the depth they are currently at, he launches one anyway, which quickly explodes. Immediately afterwards, they find Templeton dead of poisoned coffee. The episode is only well begun and most of the guests have been killed off!
Nelson and Stroller carry Templeton down to the rapidly filling Sickbay. Stroller says that he'll wait for Nelson outside--and promptly vanishes, and does not answer Nelson's calls. Nelson's search for him ends when he falls off a loosened ladder, narrowly missing serious injury. Stroller belatedly arrives--to face Nelson's gun. Nelson is certain that a woman could not be responsible for what's been going on (well, this was the mid-60's) which leaves Stroller as his prime suspect.
Nelson apparently was threatening Stroller with an empty gun--after they return to the Control Room, he pulls two ammo clips out of the small arms cabinet, hands one to Crane, and loads his gun. Rather stupidly, he left the cabinet hanging open. Ava chooses her moment to slip a gun into her purse, but not well enough--Stroller spots her, but says nothing at the time.
They have estimated their location, and the location of the Peace Ship--and Nelson guesses that the purpose of the murders is to take over the Seaview and destroy the Peace Ship. He attempts to prevent Seaview's weaponry from being used by shooting out the torpedo control panel (shouldn't there be one down in the Missile room, as well?) and then sends Crane to deal with the nuclear missiles on board. Incredibly, instead of disarming the warheads, Crane actually explodes something in the missile launch tubes, which seems like an appallingly dangerous thing to do. Stroller finally confronts Ava about the gun, which she claims that she took for protection. No explanation for why she didn't ask Nelson for it--Nelson trusts her enough to let her keep it.
On his way back from the Missile Room, Crane sets off a booby-trapped hatch, and a grenade blows up right in his face. They should have shown him slamming the door shut before it went off, because it's ridiculous to think that he could have survived that with no more than a concussion and a cut on his head. Tracy, the computer expert, who has been more or less dithering about (and will resume dithering shortly) suddenly indicates that she's trained in first aid, having assisted her father, an old country doctor. There's a silly little moment as she's giving Crane a mild sedative (which sounds a little dangerous for a man with concussion). She spoon-feeds it to this grown man. (I almost expected her to say, "Now, open wide....")
Ava goes to her cabin to rest, and the remainder go back to the Control Room. It's not clear just how much time passes. Nelson and Tracy have been trying to trace the loss of power; Crane and Stroller have fallen asleep. Ava calls Nelson, but her call is cut off. They rush down and find her dead, although in her case the method is never explained. Possibly she was hit from behind. The suspects are now down to two.
Nelson appears to be the next victim--and I really wish they had dealt with this differently. We all know they're not going to kill Nelson off, so it was obviously a trick. The audience would have been kept guessing a while longer if Nelson had been reported in a coma, or perhaps paralyzed. And just who rigged that panel? Was it Nelson, or had Nelson spotted the killer's tamperings and rerouted it for his trick?
Almost immediately, Stroller is killed off, in a quite nasty fashion--pinned between the crash control doors. The confrontation between Crane and Tracy is excellent. Crane is convinced that Tracy is guilty. Tracy presumably thinks that Crane is the guilty one, but she doesn't say so, and in fact, her flight from him might have pure panic, not fear that he was about to kill her. (As I've noted, she's a dithering sort, but the look on Crane's face could have frightened anyone.) After Crane corners her, the two of them find themselves locked in--obviously there is still someone else left alive on the Seaview.
The observant viewer could probably guess who the killer was by looking at the view of him from the waist down. He prepares a torpedo for launch down in the Missile Room--and seems to leap instantly to the Control Room, where he removes the shot-up firing panel--which is revealed to be a fake, the undamaged real panel beneath it. I'd like to know just when he managed that little switch.
Crane jumps the man before he can fire on the Peace Ship, but, weakened by concussion, he's knocked down (but not out). It's left to Nelson to deal with the man, coming out of hiding with a nice "Gotcha!" expression on his face.
Nelson and Crane are left explaining matters to the sole survivor, who's rather put out that she wasn't told about Nelson's trick, although she was obviously still a suspect at the time. Crane, quite cheerfully, confesses that he came close to killing her.
An interesting and unusual change of pace for this series.