Four men, members of a Welsh rugby union club fly to Paris, as part of an outing, for the weekend to see Wales play France in the Five Nations Championshipa match that will decide the Grand Slam title. One of the party is Caradog Lloyd-Evans (Griffith), who briefly served in occupied Paris during World War II. Caradog pays for his son Glyn's air ticket on the proviso that Glyn comes on a 'pilgrimage' to find his 'little butterfly' who he spent a short romantic period with during the war. This pilgrimage is successful and although the right place is found, it is no longer the innocent bistro of his youth but one of many strip club joints. Mr Lloyd-Evans mistakes a young girl in the club (who is a spitting image of his 'little butterfly') for the real thing and finds that the girl is actually the daughter of his old flame. He is (naturally) disappointed but Glyn (Dewi Pws Morris) gets himself acquainted with the daughter while Caradog reminisces with his 'butterfly'. A quick call to the hotel brings the entire tour party to the club where fun and frolicking takes place led by Mog (Davies). Some locals take exception and a mass brawl starts which ends with the arrival of the police. All are arrested except Caradog (protected by his 'butterfly') and Glyn (hidden in the daughter's room).
Sion Probert plays camp boutique owner Maldwyn Pugh, who is the only one of the original four who makes the start of the match, because Mog (Davies) is still in jail because he was considered the ringleader (the others were released), Caradog is with his 'butterfly' and Gwyn is trying for his own sexual 'Grand Slam'. Mog is eventually released halfway through the first half of the match, but when he finally arrives at the stadium, the match has seconds to go and Wales unfortunately lose. Caradog is found collapsed near the table, and his old flame thinks he has died, but her daughter fires a soda siphon on his face, and the story ends with Caradog warning his son of the dangers of overseas travel.
The film is iconic in Wales, being broadcast many times since it was made. It is also popular in Australia.
This film marked the penultimate film or television appearance of actor Hugh Griffith who died in 1980.