There's a problem at school, and of course, our Prof (short for professore [professor, teacher], the affectionate, abbreviated term students use in addressing a teacher) has to investigate.

During questioning, the passengers seem to agree about one person getting back on the bus during the rest stop: Luca's sister. Buttafuoco and Barbagallo finish up checking the luggage, but they come across an extra bag.

At the Olivetti home, it's Christmastime, and there's an air of celebration among the members of the extended family. Karen, who is listening in, is engulfed by feelings of nostalgia for her mother.

Manara is back in the saddle. Raimondi leaves for Rome after a frank conversation with Luca, and another one with Marta. Quattroni has some important information weighing on him, and goes to talk to Manara.

The election results have come out, and they're disappointing. Grazia gives Adriano all her support, and this gives him the hope he needs to push ahead with his dream.

Manara, and Raimondi meet with their chief, Casadio to discuss the case and Manara's status, but the meeting has an unexpected outcome. Meanwhile, the only real suspect is Luca's sister!

Karen continues to do her work for the Americans, spying on Olivetti and the Community Movement. Dalmasso and Mauro are seriously trying to undermine Olivetti's efforts to get elected and to bring him to his knees financially.

Tiziana is confused because she has no clue that she might be a suspect. Raimondi, who shows up with Rivera, is confused because he doesn't realize Tiziana is Luca's sister. Poor Tiziana!

The laboratory team is all excited as they come up with some great new innovations. They need more funding, however, and decide to demonstrate the results directly to the board.

The investigation gets going. Luca, officially still off duty, had just come to meet his sister at the bus, but naturally, he can't help getting involved in the case, which causes a bit of confusion on the team. Rosmini comes to examine the victim.

The factory in Pozzuoli is a success, but the company has taken financial risks, which may be why Adriano is having nightmares. Dalmasso is closely following Adriano's movements.

Marta confides in Ginevra about Luca's declaration of love. Meanwhile, a bus arrives in town, and, as the passengers are getting off, there's a problem. At the same time, Luca is giving Ada a hard time as she drives him to the bus stop to meet his sister.

Adriano goes to Rome to try and get financing for his project. He is unpleasantly surprised to have to deal with someone he knows very well and with whom he had had a falling out.

Luca has resolved the case, and Marta arrives with Fabrizio to take over from him. But who could have predicted what happened next?

Karen's report to her superiors about Olivetti's plans prompts some serious spying on his operation by the U.S. Mario Tchou explains his ideas to the new team of young engineers.