As the sun rises over a mountain range, Professor Lundgren and an assistant watch and marvel at its power. The professor intends to harness the power in increasing quantities using his solar generator, a parabolic dish that focuses sunlight onto a heat exchanger. They start the test, with another test planned for the night to power the town below.
Lady Penelope packs in her mansion. As she has Parker haul her luggage to the car, she tells him to consider it a vacation and gives him permission to dress casually, which Parker takes advantage of as rain begins to pour outside. He has no worries, though, as "the sun always shines at Monte Bianco."
Signore Puccini is excited that the hotel will be full that night as tourists flock to see the first town powered by the sun. But Bruno the elderly porter is not happy about the extra work, saying it's against nature and giving a dire warning that the sun will have its revenge. Puccini is dismissive.
En route to Monte Bianco in FAB1, Penelope and Parker try to listen to the radio, but there's interference on all channels. Penelope assumes it's because they're surrounded by mountains.
On Tracy Island, Jeff and Brains discuss the solar generator, which Brains considers a real breakthrough.
As Lundgren and Mitchell prepare the generator, a storm begins to loom on the horizon. FAB1 arrives at the hotel in a thunderstorm. Penelope takes it in stride, saying the English are always prepared for rain as she wears galoshes. However, Parker is dressed in sandals and his socks are soaked. Bruno tells him he'd better change before he floods the hotel. They dress for a costume ball that night, where Puccini announces they're about to receive power from the sun. On cue, the lights come up even in the driving rain. But as they eat dinner, the lights dim occasionally. Puccini has the musicians play to distract the crowd.
At the generator, a lightning bolt strikes the tower holding the dish. A second strike destroys the lightning conductor. Quickly, a third lightning strike damages the tower. Lundgren orders the power to be cut, frightening the guests at the ball as they're plunged into darkness. Lundgren decides to go up to the dish, but a final bolt sends the reflector dish falling down the mountainside. Lundgren is injured. The dish, lodged halfway down the mountain, reflects the moonlight onto the town as the clouds clear. Bruno approaches Penelope at her table and warns of impending disaster. Puccini stops him from talking further, but Penelope is convinced. She slips from the ball into FAB1. The radio doesn't work, though. She needs to be clear of the mountains, so she decides to drive FAB1 out to sea. When she passes a yacht, a drunk on the railing thinks he's seeing things. She's finally able to contact International Rescue and quickly explains the situation, which Brains and Jeff immediately understand the gravity of. Thunderbirds 1 and 2 are dispatched, with Alan and Brains joining Virgil on T2.
Penelope instructs Parker to keep everybody inside, then heads for the power station. Parker decides to set up a Bingo parlor to keep the people from realizing the danger and panicking. Bruno wakes Puccini to help calm the guests, posing as an English lord to gain authority.
Thunderbird 1 overflies the generator, sending pictures to Thunderbird 2. Brains believes the dish can be tilted upward. When they arrive, Brains insists on going to the dish himself to inspect it, over Alan's objections. Virgil allows it but tells him to take no risks. Brains is lowered in a heat suit and a harness. But he finds the harness too much of a hindrance and unclips it. Climbing higher on the dish, he asks for the high-tensile cable with an electromagnetic clamp. After Brains is clear, Virgil tries to tilt the dish but the reflector is too heavy. Brains checks the rotation gears and finds them jammed. He wants to use a laser to unjam it but there's little time left. The hotel is beginning to smoke under the reflected sun, with the tar roof bubbling. That gives Brains an idea. He instructs Thunderbird 1 to use a smoke generator he had recently installed to temporarily block the sun. After Brains clears the gears, Thunderbird 2 successfully lifts the dish backward. A sudden landslide carries the suit and the dish down the mountainside, alarming Alan and Virgil. Brains is actually safe, having taken off the suit because it was too hot and awkward.
Penelope asks for an explanation from Parker about his "lord" masquerade and forgives him his deception. She wonders what could engross the guests so much that they wouldn't notice the danger. When Parker says it was Bingo, she admits she doesn't know the game. She suggests they head down to the beach. Parker has anticipated this, opening his cloak to reveal his swimsuit.