At a US air force base, a large hangar opens and an RTL-2 jet rolls out onto a runway. In the control tower, the Commander and his staff watch the plane move; and whilst it looks fine, well, they said that about the previous one. In the cockpit, the crew open up their top-secret orders - only they know where they need to fly to. Captain Savidge says he'll make one last call to the control tower.
The Commander tells him that once he's in the air, he's to maintain total radio silence until landing at his destination. He should only break the silence in the case of an extreme emergency. Savidge says he'll be glad when they deliver their cargo - two large and highly-powerful missiles. As the plane starts to take off, one of the crew has his radio on. A song performed by the Cass Carnaby Five, live from the Paradise Peaks Hotel, begins to play...
The RTL2 takes off without any problems. Dangerous Game is an international hit; and on Tracy Island, the whole family is listening to it as well.
Even in space the song is on, as John and Alan are listening in Thunderbird 5 as they prepare for their monthly duty change-over. But, suddenly, they hear an emergency call - from the RTL2 jet!
The crew call the control tower: they're under attack, from three enemy fighters with oval markings. It's the third time it's happened. the Commander says...and then, contact with the RTL2 is lost. Badly-damaged, the jet's gone into a dive and crashed...
Later on, after he's returned to Tracy Island, Alan asks Tin-Tin if it was a coincidence that Dangerous Game was playing on Radio Maxwell, when the RTL2 was attacked? Scott questions why that would be important, as the song must've been broadcast a million times. Alan points out that Dangerous Game happened to be playing live when the two other RTL2 crashes occurred. Brains says he'll have John send a recording of the song down from TB5, so he can analyze it. Learning that the Cass Carnaby Five are currently performing at a deluxe hotel known as Paradise Peaks, Jeff decides to send Penelope there to investigate...
Penelope's just settling down for tea, in her outdoor summer house, when she receives Jeff's call via her teapot radio. He explains that he wants her to go to the Alps, where Paradise Peaks is located. Parker's about to have the afternoon off to take out Lil the cook - and looking forward to it! - but Penelope asks him instead to get ready: they're going on a mission, and she needs him to use his connections in the theatre world...
To that end, Parker pays a call on an old friend of his called Maxie, who runs International Artistes Management Ltd. He wants a singer called Wanda Lamour to be sent to Paradise Peaks to perform. Maxie's reluctant to send someone he's never even heard of, to such a place...but Parker then rather cagily mentions that he was in the United States the other week, where he met one "Punchy" Patterson. Maxie becomes alarmed on hearing that name - he thought Patterson was in jail - but Parker says a guy as muscular as "Punchy" was never going to stay behind bars for long. And, he told Parker, he had some...unfinished business. Maxie gets the picture, and makes a deal: he'll have Wanda Lamour sent to Paradise Peaks; but in return, Parker will keep his mouth shut, so "Punchy" doesn't find out where Maxie is!
Later on, as Penelope's getting ready - including having her Wanda Lamour guise in readiness - out on Tracy Island, Tin-Tin's preparing too. She's going on the mission with Penelope, and leaves on her small jet, Ladybird, as Alan watches from the runway....
As Penelope awaits the departure of her own flight, she contacts Jeff and asks about any new developments. Jeff advises that Brains is working full-time on the song-recording John sent him. Alan's helping him; and Brains describes how repeated melodic patterns in the music could be used to impede the efficiency of the aircraft. Each time the RTL2 crews reported in, Alan states, they said they were under attack, and didn't report they had malfunctions. This sets Brains to wondering if the music could contain some kind of code. Alan says they'll have to try and crack it, as Penelope and Tin-Tin will soon arrive in the Alps - and they'll need all the help they can get...
As she and Tin-Tin take a cable car up to the Paradise Peaks hotel, Penelope says she'll pretend to have a sore throat so she won't be asked to perform, meaning they can both go about their investigation. Later on, they're in the audience as the Cass Carnaby Five perform Dangerous Game. Tin-Tin finds it hard to believe that the group would have anything to do with the destruction of those military jets. As the music concludes, Cass Carnaby, takes to the mic and introduces the whole room to Wanda Lamour - who, he says, cannot perform tonight due to a sore throat!
Backstage, Tin-Tin manages to have a few words with Cass. She likes the song, but doesn't he ever get tired playing it over and over again? Doesn't he want to vary it? Cass says he leaves stuff like that to their arranger, Mr. Olsen - who, it happens, is behind them and, after checking his watch, says he needs to take care of a few things. And he needs to have a word with Cass. As he leaves, Cass explains that while Olsen knows the business, he can be a bit temperamental - he sometimes makes last-minute changes to their music, before they perform it...
While everyone is out on the dance floor, Wanda gets a chance to dance with Olsen. He says it's strange their paths haven't crossed before, and he knows very little about her. ("High society keeps me pretty busy," says she.) Wanda says that he's also a man of mystery...but as the current tune ends, he says he has to leave. Penelope hasn't learnt as much as she would've liked...
Parker then calls her over to the bar - he's gotten part-time work as a bouncer, as he thought he'd be more use to her at the hotel. She admits she and Tin-Tin are having very little success. Later on, the three regroup in Penelope's room, where Parker says he overheard Olsen talking to Banino, the head waiter, about a message he was expecting. Penelope thinks they should follow this up, and finds out Olsen lives around the other side of the mountain.
On Tracy Island, Brains continues experimenting with the recording. He tells Jeff there's some kind of electronic pattern in the music, and it's looking quite likely to have been used as a code. Jeff then takes a call from Tin-Tin: they haven't discovered anything yet, but believe Olsen could be the man they're looking for. They don't know if he's working alone, or if Cass' group is involved. They'll call in again tomorrow, after they've visited Olsen's place...
Next day, as they quietly approach Olsen's home, Tin-Tin and Penelope (both on skis) see him using what looks to be some kind of computer. Strange symbols appear on its screen, which Tin-Tin films using a hidden camera. The symbols change into English text - stating when the next RTL2 jet is going to take off!
But Olsen happens to see tracks in the snow outside his chalet, and uses a telescope to see that two people are heading away. He phones Banino, who finds out the "singer nobody's heard of" is causing trouble, so he'll deal with her. Nearby, though, Parker happens to overhear...
Banino drives out to the mountain, and prepares to shoot Wanda and her friend...but, before he can, Parker charges into him, then grabs him and removes him from his car, as they both roll haplessly down the mountain...
They soon turn into a large snowball, but eventually come to a stop when they roll into a large rock and the snowball breaks on impact. Penelope and Tin-Tin hurry over, and see that Parker's all right. Banino is unconscious. That gun-shot they heard, Penelope says, was clearly meant for them; and they hurry back to the hotel. They need to contact Jeff, and quickly; and show him the film Tin-Tin recorded.
Brains, coming into the lounge to take a look, realises at last what it is - it's a cham-cham! (A who?) He hurriedly explains that a cham-cham's an electronic machine that's sensitive to ultra-sonic harmonics and microtones. Now that he knows the technique they've been using, he can break their code in no time. Jeff advises Penelope to wait for his next message. He then has Scott get in touch with Washington DC; they've got to save that plane. At Matthews Field, the next RTL2 jet is heading towards the runway...
Word reaches there that International Rescue has sent a warning - but the Commander refuses to believe it, and likewise refuses to stop the RTL2 from taking off on schedule, which it presently does. Back on Tracy Island, Alan says Brains has cracked the code, and composed an altered version of Dangerous Game - one which will change the message. Jeff says it's up to Penelope and Tin-Tin now - only they can save the RTL2 from destruction...
At Paradise Peaks, Olsen tells Cass he's changed the tune of Dangerous Game slightly, and wants him to play it a different way. Cass questions why he has to keep changing it, but Olsen reminds him of their contract - they agreed to do things his way. Cass says he'll do it, and Olsen leaves...but Tin-Tin then appears, and tells Cass if they're going to change the tune, to do it her way. Why does everyone wants to change Dangerous Game? She says he'll just have to trust her...
Scott is taking off in Thunderbird 1 - bound for Matthews Field, to prove that International Rescue really is involved. Behind him, his family tunes in to Radio Maxwell, as the Cass Carnaby Five are about to play. Brains says his arranged version of the tune will divert the enemy fighters...but only if Penelope can make the band play it. The performance begins - at a faster tempo, and one of the band is using a flute, rather than a saxophone.
At an enemy base, a Colonel is listening to the music on a radio, but he and his Lieutenant are using a computer to extract information from the tune - the RTL2 jet's flightpath - and the Colonel has three of his fighter jets prepare for launch...
Cass and the band continue to play...but then, Wanda Lamour steps up to the mic. She begins to sing. The tune changes...and so does the hidden code...
At the enemy base, the Lieutenant says the message is changing: the co-ordinates of the jet they want to destroy is now at point 629668; and the Colonel so advises his three pilots to take off, and shoot that aircraft down!
As Wanda continues to sing, Tin-Tin lets Parker know that Olsen is watching. As the song finishes, Jeff is ordering Virgil and Alan to get going in Thunderbird 2 - Penelope and the others could be in great danger...
Scott lands TB1 at Matthews Field; and through a loudspeaker, he tells the control tower staff that International Rescue tried to warn them about the danger their RTL2 jet was in from an enemy attack - but they did not listen. However, IR has now diverted three enemy fighters, which will fly over the outskirts of the airfield at any moment. Sure enough, the tower staff picks up three unidentified contacts...and, realizing what's happening, dispatches their own jets to shoot them down. The Commander's reluctant to admit that International Rescue has done it again. (Be that as it may, though...)
Early in the morning, Penelope, Parker and Tin-Tin leave Paradise Peaks, knowing they must get away from there quickly (albeit not having paid their bill) - Olsen's somewhere out there, and surely intent upon killing them.
But Olsen's observed their departure; and as they head away from the hotel in a cable car, he shuts off the power to the cable-car system. He then uses a blowtorch to start cutting through the wiring - and is suddenly confronted by Cass Carnaby! He heard the alarm go off, and now tries to stop Olsen...
Penelope tells Jeff they need help, and learns TB2's already on the way...
Despite Cass' desperate efforts to stop Olsen cutting through the cable, it's too late - and the cable snaps!
TB2 is at last on the scene - and Alan spies a cable car, rapidly descending down the mountain...so fast the cable must've been cut...
Virgil dives TB2 down the mountain in pursuit, towards the runaway cable car. The magnetic grabs Alan fires won't hold, and slide off. Virgil tells Penelope Parker must help them - and, opening the car's door, heads up to the roof...
Virgil releases several cables and, using an umbrella Penelope passed up to him, Parker connects each one to the cable car. Virgil then uses TB2's retros - the force of which causes Parker to slide off! The cable car comes to a safe stop just in time, and Parker has also survived...thanks to that umbrella...
At Paradise Peaks, it's very late. Penelope and Virgil, both in dinner-dress, are at a table; and he queries if the hotel staff will know they're from International Rescue? Penelope doesn't think so...although Cass seems quite interesting in knowing how they knew how there was a code, and how they broke it. Cass, playing the piano, asks about Tin-Tin. Penelope says that she's outside, getting some air with Alan. Cass knows she's going home tomorrow; but, he sighs, that's show business! Outside, Alan asks if she's all cut-up about leaving Cass and his band behind. She says she's getting use to this "hello, goodbye" thing, and that it's always nice to get home again...