Up the Line is my favorite Silverberg novel. Unlike his weighty explorations of cultural, social and psychological issues, UTL is a romp, a comedy, and a light-hearted look at the absurdities and paradoxes of time travel, perhaps even a good-natured dig at his fellow SF writers who have mined this mother lode of material so completely. And it also gives the author an opportunity to show off his expertise on the history of the Byzantine Empire.
The protagonist is a member of the Time Travel Bureau. His job is to escort scholars and tourists on brief visits back into the past. He makes sure they have their vaccinations, have mastered their hypnosleep language courses, and that their costume is appropriate for the era in history they are visiting. He coaches them on the local customs, appropriate behavior, and then watches out that they keep a low profile and don’t get into any trouble while they are there. It is also essential that no one in the past realizes they are visitors from the future. The Bureau is very keen on making certain no time travel paradoxes occur, and if they do happen, some fancy time hopping is needed to set things right.
You learn a lot of interesting things while Up the Line, the term for going back in time. Down the Line is forward, with the time stream, towards your own present. For example, every time our hero takes a party to visit the Crucifixtion of Jesus, (a very popular destination)he notices the crowd is larger. He realizes most of the spectators are from the future! He is repeatedly told that if he should ever run into one of his colleagues in the past, or even himself, he should never show any sign of recognition. One of the two may not be aware that the other is there from his future. It could cause problems.
He also realizes that some of his colleagues at the Bureau are doing a bit of unauthorized time-hopping off-the-clock, so to speak. Some have girlfriends in the past, do a little smuggling, commit crimes and even take extended vacations up the line, all without the knowledge of the Bureau, which would find such unauthorized excursions highly illegal. With their knowlege of temporal voyaging, they can pick places and eras where it is unlikely paradoxes will be generated. One of his fellow workers has a luxurious villa in ancient Byzantium, and a profitable plantation business well stocked with slaves. It’s his home away from home. However, he spends so much time there people at headquarters are starting to notice he seems to be aging prematurely.
It is during a visit to this pleasure palace that our hero finds himself involved with local palace intrigue, the Blue and Green Chariot races, and eventually winds up bedding the notorious Empress Theodora, the most beautiful and most wicked woman in the world, maybe in history, who he later finds out is his great-great…great grandmother, and he is actually his own ancestor.
He gets deeper and deeper into trouble, causing paradoxes and worse when he tries to double back and repair the damage he’s done to the time stream. He not only finds himself running into himself in the past, but even fighting himself. At one point there are several copies of him in the room, each with different knowledge of what is going on.