Tobit was a righteous Israelite of the house of Naphtali. He was grieved by the idol-worshipping of his brethren, and he lived in Ninveveh, which was a hostile place. (He didn't live too long after Jonah's time, and Jonah tried to flee rather than preach in Nineveh.) Tobit would give to the poor, and bury bodies he found lying outside the walls of Nineveh, which some townspeople didn't appreciate. In fact, the king took all his possessions and exiled him. After king Sennacherib dies, Tobit is allowed to return to Nineveh. At one point, while he's sleeping outside, a bird drops dung in his eyes, rendering him blind.
Around this same time in the land of Media is a widow named Sarah, grieved, because she had been given in marriage seven times and all of her husbands have died on their wedding night, killed by the evil spirit Asmodeus. She's childless and an only child, so she prays for a husband who can give her children, and Tobit prays for his sight to return. God sends the angel Raphael in disguise to answer their prayers.
Chapter 4 is where some spiritual worth comes from this book, as it is mostly Tobit passing his wisdom to Tobias before he dies.
3 And when he had called him, he said, My son, when I am dead, bury me; and despise not thy mother, but honour her all the days of thy life, and do that which shall please her, and grieve her not.
4 Remember, my son, that she saw many dangers for thee, when thou wast in her womb: and when she is dead, bury her by me in one grave.
5 My son, be mindful of the Lord our God all thy days, and let not thy will be set to sin, or to transgress his commandments: do uprightly all thy life long, and follow not the ways of unrighteousness.
6 For if thou deal truly, thy doings shall prosperously succeed to thee, and to all them that live justly.
7 Give alms of thy substance; and when thou givest alms, let not thine eye be envious, neither turn thy face from any poor, and the face of God shall not be turned away from thee.
8 If thou hast abundance give alms accordingly: if thou have but a little, be not afraid to give according to that little:
9 For thou layest up a good treasure for thyself against the day of necessity.
10 Because that alms do deliver from death, and suffereth not to come into darkness.
11 For alms is a good gift unto all that give it in the sight of the most High.
15 Do that to no man which thou hatest: drink not wine to make thee drunken: neither let drunkenness go with thee in thy journey.
16 Give of thy bread to the hungry, and of thy garments to them that are naked; and according to thine abundance give alms: and let not thine eye be envious, when thou givest alms.
Raphael claims to be Azariah, a kinsman of Tobit's, and he accompanies Tobias on his journey to Media to collect some money his father had left there years ago. At one point, while they're washing by a river, a giant fish tries to eat Tobias, but he kills it, and by order of Raphael, he removes its heart, liver and gall bladder.
Raphael lets Tobias know he's going to give him to Sarah to marry, but Tobias is nervous, because he knows all of her previous husbands had died. Raphael instructs him to make a perfume with ashes and pieces of the fish's heart and liver, and it will keep Asmodeus away.
Sarah's parents are thrilled when Tobias arrived, a kinsman their daughter can marry. Tobias prepares the fish-organ perfume, and Asmodeus flees from the smell. Raphael then binds him. Tobias and Sarah are then free to consummate.
The next morning, Sarah's parents are afraid they'll have another dead man to remove from their daughter's bedchamber, but all is well. Raguel, Sarah's father, sings his praises unto God. He gives half of his possessions to Tobias & Sarah, and pledges he will inherit the other half when Raguel dies. Tobias, Sarah, and Raphael head back to Nineveh.
Back at home, Anna (Tobit's wife) wails that her son is dead, while Tobit tells her to wait. As Tobias approaches, Raphael tells him to use the fish's gall bladder to anoint his father's eyes, and his eyesight will return. Anna runs to meet her son and rejoices, and when Tobias comes up to his father, he anoints his eyes, and Tobit's eyesight is restored.
As the family celebrates in Chapter 12, Raphael gives some advice to Tobit before revealing his true identity.
8 Prayer is good with fasting and alms and righteousness. A little with righteousness is better than much with unrighteousness. It is better to give alms than to lay up gold:
9 For alms doth deliver from death, and shall purge away all sin. Those that exercise alms and righteousness shall be filled with life:
10 But they that sin are enemies to their own life.
Raphael then reveals himself and says that he came to answer their prayers to God: Tobit, for his sight to return, and Sarah, that she might marry a husband who lives.
15 I am Raphael, one of the seven holy angels, which present the prayers of the saints, and which go in and out before the glory of the Holy One.
16 Then they were both troubled, and fell upon their faces: for they feared.
17 But he said unto them, Fear not, for it shall go well with you; praise God therefore.
18 For not of any favour of mine, but by the will of our God I came; wherefore praise him for ever.
19 All these days I did appear unto you; but I did neither eat nor drink, but ye did see a vision.
20 Now therefore give God thanks: for I go up to him that sent me; but write all things which are done in a book.
21 And when they arose, they saw him no more.
22 Then they confessed the great and wonderful works of God, and how the angel of the Lord had appeared unto them.
Chapter 13 is Tobit praising God.
7 I will extol my God, and my soul shall praise the King of heaven, and shall rejoice in his greatness.
8 Let all men speak, and let all praise him for his righteousness.
9 O Jerusalem, the holy city, he will scourge thee for thy children's works, and will have mercy again on the sons of the righteous.
10 Give praise to the Lord, for he is good: and praise the everlasting King, that his tabernacle may be builded in thee again with joy, and let him make joyful there in thee those that are captives, and love in thee for ever those that are miserable.
11 Many nations shall come from far to the name of the Lord God with gifts in their hands, even gifts to the King of heaven; all generations shall praise thee with great joy.
12 Cursed are all they which hate thee, and blessed shall all be which love thee for ever.
13 Rejoice and be glad for the children of the just: for they shall be gathered together, and shall bless the Lord of the just.
14 O blessed are they which love thee, for they shall rejoice in thy peace: blessed are they which have been sorrowful for all thy scourges; for they shall rejoice for thee, when they have seen all thy glory, and shall be glad for ever.
15 Let my soul bless God the great King.
Chapter 14, as Tobit is getting old and near death, he tells Tobias to move his family back to Media, for the prophet Jonah had foretold that Nineveh would fall. Tobit dies at age 158, and Tobias buries him, and then Anna dies, and Tobias buries her next to Tobit. Tobias, in his old age, hears about the destruction of Nineveh while he's living in Media, and then he died at age 127.
I've read that Tobit was moved to the Apocrypha because there were too many doubts about when it was written, think it was around 2nd century BC even though the time period is 8th century BC, but they've since learned that it actually was written in near the time period it takes place.