Ciaran vic Cairpre, nephew of a Dál Riatan warleader in Scotland, is a hostage of the Romans. When the ursurper Constantine leaves Britain with most of the army in 408 AD, Ciaran, left without protection, becomes a slave by the schemes of the freedman Orontes Vahan, bound on revenge on the northern tribes.
As it turns out Ciaran has a talent to work with horses, the Roman patrician Julius Pollienus buys him and supports him to make a career as charioteer in Rome. Ciaran is fascinated by the Roman culture, but when Pollienus wants him to join the conspiracy against general Stilicho, Ciaran refuses, and his life is no longer safe. Moreover, Orontes has followed him to Rome.
Ciaran, realising that his responsibility lies with his people, flees Rome. (there's a major disconnect in logic here--what does "his responsiblity to his people have to do with not conspiring against the general?)
Upon his return to Britain, he learns his rival, the Pictish chief Eoghanán mac Enfidaic, has married his betrothed and taken over the rule of the Dál Riatans. Ciaran tries to regain his position with help of his contacts to the Romans, but increasing troubles with the Saxon raiders under their leader Aescwyne bind the remaining Roman soldiers to the southern shores. Then Pollienus and Orontes return to Britain to claim the escaped slave.
The Saxon invaders may be Ciaran's only chance, but the price of this alliance could prove too high.
Like many writers of historical fiction you've confused lovingly detailing the place and time with creating a plot. Or at least one here.
Back to basics. Then festoon as needed.