It takes place during the First Age of Middle-earth, about 6500 years before the events of his most famous book, The Lord of the Rings.
Tolkien wrote several versions of their story, the latest written in The Silmarillion.
Beren and Lúthien are also mentioned in The Lord of the Rings.
The first version of the story is the Tale of Tinúviel, which was written in 1917 as a part of The Book of Lost Tales.
During the 1920s Tolkien started to reshape the tale and to transform it into an epic poem which he called The Lay of Leithian.
He never finished it.
The story and the characters reflect the love of Tolkien and his wife Edith.
Particularly, the event when Edith danced for him in a glade with flowering hemlocks seems to have inspired his vision of the meeting of Beren and Lúthien.
On Tolkien's grave, J. R. R. Tolkien is referred to as Beren and Edith is referred to as Lúthien...
Song of Beren and Lúthien
The leaves were long, the grass was green,
The hemlock-umbels tall and fair,
And in the glade a light was seen
Of stars in shadow shimmering.
Tinúviel was dancing there
To music of a pipe unseen,
And light of stars was in her hair,
And in her raiment glimmering.
There Beren came from mountains cold,
And lost he wandered under leaves,
And where the Elven-river rolled
He walked alone and sorrowing.
He peered between the hemlock-leaves
And saw in wonder flowers of gold
Upon her mantle and her sleeves,
And her hair like shadow following.
Enchantment healed his weary feet
That over hills were doomed to roam;
And forth he hastened, strong and fleet,
And grasped at moonbeams glistening.
Through woven woods in Elvenhome
She lightly fled on dancing feet,
And left him lonely still to roam
In the silent forest listening.
He heard there oft the flying sound
Of feet as light as linden-leaves,
Or music welling underground,
In hidden hollows quavering.
Now withered lay the hemlock-sheaves,
And one by one with sighing sound
Whispering fell the beachen leaves
In the wintry woodland wavering.
He sought her ever, wandering far
Where leaves of years were thickly strewn,
By light of moon and ray of star
In frosty heavens shivering.
Her mantle glinted in the moon,
As on a hill-top high and far
She danced, and at her feet was strewn
A mist of silver quivering.
When winter passed, she came again,
And her song released the sudden spring,
Like rising lark, and falling rain,
And melting water bubbling.
He saw the elven-flowers spring
About her feet, and healed again
He longed by her to dance and sing
Upon the grass untroubling.
Again she fled, but swift he came.
He called her by her elvish name;
And there she halted listening.
One moment stood she, and a spell
His voice laid on her: Beren came,
And doom fell on Tinúviel
That in his arms lay glistening.
As Beren looked into her eyes
Within the shadows of her hair,
The trembling starlight of the skies
He saw there mirrored shimmering.
Tinúviel the elven-fair,
Immortal maiden elven-wise,
About him cast her shadowy hair
And arms like silver glimmering.
Long was the way that fate them bore,
O'er stony mountains cold and grey,
Through halls of ireon and darkling door,
And woods of nightshade morrowless.
The Sundering Seas between them lay,
And yet at last they met once more,
And long ago they passed away
In the forest singing sorrowless.
The Tale of Beren and Lúthien
After the ruin of his land in the Battle of Sudden Flame the Man Beren fled into the elvish realm Doriath. There he met the Elf-maiden Lúthien and they fell in love with each other.
Thingol, the father of Lúthien, did not want his daughter to marry a mortal man.
Therefore he asked Beren for a Silmaril, one of the hallowed jewels which the Dark Lord Morgoth had stolen from the Elves, as the bride price.
With the help of Huan and Finrod Felagund, Beren and Lúthien defeated Sauron and came to Angband, where they stole a Silmaril from Morgoth's crown.
The unfinished poem ends when they encounter the wolf Carcharoth at the gate of Angband.