There is a mysterious plant growth spreading all across Italy... the Seaview and her crew are investigating. Crane is out in the flying sub scouting about, Nelson tells him over the intercom he should be right over some small towns and villages, and Crane informs him there is nothing but dense jungle below! Nelson orders him back aboard.
Meanwhile all the crew are hot and sweaty (the famous voyage front sweat marks) apparently there seems to be a problem with the reactor controls which has sent the temperature soaring.
Sharkey goes to the reactor room and asks the "new man" named Keeler what's going on. Keeler fobs him off with a yarn about a defective control and then gloats that he fixed the problem. Sharkey reminds him it might be a good idea to fix the problems before they cause serious malfunctions next time. As soon as Sharkey leaves Keeler produces a nifty little box with nifty little men inside (complete with silver faces) runs his hand over them and...pow they become life size!
Crane has returned to the Seaview and asks why its so hot only to be informed there was a problem but its all under control now (yeah right) Nelson retires to his lab to knock up a wonder weapon that he can hook up to the Seaviews laser so it can destroy the plant growth when they reach the Italian coast.
Keeler (an alien, naturally) and his silver faced jungle fighters have taken over the life support room and are busy changing the ships atmosphere to one of pure carbon dioxide. Soon all the crew become sleepy and start to collapse. Nelson (who we can only presume didn't collapse because he was working alone in his spacious lab and had more air) works out what going on and finds Crane, they then start using these great little oxygen bottles. Soon the rest of the crew are back on their feet and using the emergency air bottles. Keeler who seems to have been doing nothing all this time, changes the life support controls back and then throws a piece of jungle growth from his little box into the air vent.
Here we meet the real star of this episode...the jungle! It first makes its entrance by bursting through the air vent above the radio shack and before long the Seaview control room and corridors are thick with Jungle growth! To make matters worse the Seaview has run into the plant growth, the huge foliage soon closing in around the ship rendering her helpless. The brand new underwater footage of the seaview being attacked by the jungle growth is superb!
Nelson is going to cut his way out from the outside and orders Patterson to accompany him in the flying sub, meanwhile Crane orders the chief and Kowalski to arm themselves with radiation counters and guns to locate the source of the radiation (why guns too? At that point Crane had no idea Keeler was behind it all!)
Meanwhile Nelson has had no luck down in the flying sub and to make matters worse a nasty vine drops on Patterson and starts to strangle him! The air is giving out and Patterson tells the Admiral to get out while he can (this always makes me laugh, what if Nelson took him at his word and just said "thanks for all your help over the years and goodbyeee" then left him to die) Nelson manages to free him with a cutting torch after almost getting his hand blown off when he tried to use a ray gun due to feedback from the plant growth.
Crane,Sharkey and Kowalski are stalking the corridors with loaded guns drawn still unaware there are any villains about! The jungle looks great, the lighting (which was always 100% perfect voyage) does and amazing job of casting plant shadows on the walls and if that was not enough there is even some jungle drums in the music score!
Nelson gets in on the act here and never one to be out done, he arms himself and even dishes out grenades to men! (all while totally unaware Keeler is the villain)
After some great shoot outs in the jungle corridors Crane decides to head back to corridor 3 and asks Sharkey and Kowalski to cover him.
Crane gets away and runs into Keeler, eventually the penny drops with Keeler when Crane notices he is giving off radiation. He then realises he's an Alien and after nearly being miniaturized Nelson saves him.
The Chief and Kowalski try to move forward into the corridor where the shooting was coming from only to be captured one by one.
Keeler decides he is going to load Seaviews missiles with his plant growth and just fire them wildly, the theory being that wherever they land they will spread the Alien jungle growth (why not do this in the first place?)
After a shoot out (Nelsons turn to get shot in the shoulder this time, the crew take it in turns) and more grenade throwing Crane and Nelson save the day. All that's left of Keeler and his silver faced jungle fighter are burnt little men and bits of plant.
Nelson soon clears the ship of the growth with is gadget (i love the way these things never leave a mess behind) and he concludes that Keeler was nothing more than plant growth himself....
It is great to see all the jungle inside the Seaview! I know it's daft but this was season four!
A few spoilers: One minute Keeler wanted the crew all dead so he changed the life support system to kill everyone and later on he was instructing his men to capture key personnel to help load missiles and run Seaview.
I'm not quite sure the life support tampering makes sense.