Salvador is furious. The greatest secret of humanity has just been revealed. A secret that only a few chosen at the Ministério do Tempo knew: the cause of the Mona Lisa's smile, Leonardo Da Vinci's famous painting. And to make things worse, it's weekend. Most of the Ministry of Time employees are off duty. Including Amélia, Tiago and Afonso. With no other alternative, Salvador sends Irene and Nuno Gonçalves to investigate what is happening in the Paço da Ribeira, on October 30, 1755, two days before the earthquake that destroyed Lisbon.

Nuno Gonçalves is enthusiastic about the mission. After all, much of his work is in the Paço da Ribeira. Nuno Gonçalves sees in this mission a chance to save it. Salvador and Irene strictly prohibit him from saving a picture that is.

When they arrived in 1755, Nuno Gonçalves and Irene presented themselves to Queen D. Mariana as a newly arrived couple from Brazil, with an interest in acquiring a nobiliary title. For that to happen, they want to present the King with lost works of the famous painter, Nuno Gonçalves. D. Mariana, pleased with the offerings, invites them to sleep in the Paço da Ribeira.

That night, Irene and Nuno Gonçalves investigate the corridors of Paço da Ribeira. Suddenly, a soporific gas is released. Irene and Nuno Gonçalves fall asleep ... and wake up the next morning without knowing what happened. After a quick examination of the works of art exhibited in the Paço da Ribeira, they realize that someone changed the originals for cheap forgeries. They are facing the perfect robbery! As Paço da Ribeira will be destroyed within two days, no one will ever know of the robbery.

Irene asks reinforcements to Salvador. This one tries to summon Amélia, Afonso and Tiago. But the patrol is unavailable. With no alternative, Salvador summons Camões. He could not be happier to go on yet another mission.

Camões arrived in 1755. That night, he and Irene, already armed with gas masks, patrol the corridors of the Paço da Ribeira. They discover that the assailants are agents of the Companhia. And heading them are, neither more nor less, than Mafalda Torres and Bennet.

Camões manages to capture Bennet but this one does not surrender with life. Mafalda Torres manages to escape ... only to appear in Salvador's office. With a proposal the Secretary General of the Ministério never expected to hear.

Salvador está furioso. O maior segredo da humanidade acabou de ser revelado. Um segredo que apenas uns poucos escolhidos no Ministério do Tempo conheciam: o motivo do sorriso da Mona Lisa, o famoso quadro de Leonardo Da Vinci. E para piorar as coisas, é fim de semana. A maior parte dos funcionários do Ministério do Tempo está de folga.