I, Claudius - Season 1 Episode 6

Tiberius, aided by Sejanus, is proving to be a harsh ruler with only Germanicus able to keep him under control. However, whilst he is in Syria Germannicus is mysteriously poisoned. His widow Agrippina is able to rally public support due to his popularity and accuses Piso, the governor of Syria, of being the slayer. Despite claiming an alibi, Piso and his wife Plancina are brought before the senate to stand trial, with Martina, a star witness, being kept under wraps. In fact, Martina was the poisoner, encouraged by Germanicus's son, young Caligula, whose insolence and taste for incest with his sister are already giving his elders cause for concern. Piso claims to have scrolls incriminating Livia and Tiberius in the murder but Plancina, in an effort to save herself, stabs her husband to death and passes it off as remorseful suicide. Thus the case is closed.