 Lus want to put up her Rumanian pen-friend Ilya for the duration of a course he takes in Leuven, and Jo his female friend Nadia who just broke up with her big-spending boy-friend Stanny, but Jeanne doesn t allow guests off the opposite gender, so they decide to exchange visitors, passing Nadia for "Ilya s sister" Ilona and Ilya for Jo s nephew Jan. Since Jef desperately seeks a secret weapon (hence the Dutch title) having bet on his billiards club s annual soccer match against Gerard s anglers club, Jo -who misunderstood Lus calling him a Bucarest international, he s actually in table tennis- agrees a price to supply him to Jefs club, which he spends on Nadia s expensive taste in restaurants...