The plot runs on two afflicted people in different cities who know nothing of each other. In Nineveh, Tobit of the tribe of Naphtali, deported under the Assyrians, keeps the law scrupulously and in particular buries Israelites executed and left lying in the street, which the king has forbidden. It costs him his property and then, by way of bird droppings in his eyes as he sleeps by a wall, his sight. In Ecbatana, Sarah has been married seven times and widowed seven times on the wedding night, each husband killed by the demon Asmodeus before the marriage could be consummated. Both pray for death in the same hour, and both prayers are heard.
The mechanism of the answer is Raphael, who attaches himself to Tobit's son Tobias as a hired travelling companion under a false name and an invented genealogy, and is not identified until the end. On the Tigris a fish lunges at Tobias; Raphael has him keep its gall, heart and liver. Heart and liver, burned as a smoke, drive Asmodeus off so that Tobias can marry Sarah, who is his kinswoman; the gall restores Tobit's sight. Raphael then declares himself one of the seven angels who stand before God, refuses payment, and tells them to write the account down.
The book's interests are practical and familial: marriage within the kindred, almsgiving, honest wages, honour to parents, and above all the burial of the dead, which frames the whole narrative and returns in Tobit's dying instructions. It is didactic fiction rather than history and is unembarrassed about being a good story. The dog that goes along on the journey and comes home with them has no theological function whatever.