Rubicon is a historical novel by American author Steven Saylor, first published by St. Martin's Press in 1999.

It is the seventh book in his Roma Sub Rosa series of mystery stories set in the final decades of the Roman Republic.

Donning his toga, he crosses town to meet the matron who reveals that her son may not have been as loyal to Pompey as one may think.

After finding her son with a mysterious box of gold, Maecia shares her belief that Numerius may have been a spy.

One of the eunuchs working there reveals that Numerius did frequent the tavern, often meeting with a man named Soscarides.

With their search leading to nothing, Tiro convinces Gordianus to accompany him to Formiae and then to Brundisium to meet with Cicero and Pompey, respectively.

Stopping at Cicero’s villa in Formiae, Gordianus is surprised to see Lucius Domitius Ahenobarbus, last known to be a captive of Caesar’s.

Afterwards, Tiro and Gordianus sneak away on a stolen skiff to reach inside the city, barely escaping Caesar’s men.

Convincing Pompey to leave Davus behind in exchange for the name of the murderer, Gordianus follows the Great One aboard his ship.

Chased by Caesar’s men, Pompey demands that Gordianus tell him the murderer’s name only for the finder to confess that he did it.

Gordianus reveals that Numerius had blackmailed him with a document proving an assassination plot against Caesar—with Meto’s name on it.