Epica by Kamelot is a really great album, and one of my favorite concept albums ever (and I have listened to alot of concept albums)
And if you have the time, check out The Black Halo by the same band, it's the second part of the story started in Epica.
Based on the "Prologue in Heaven" from Goethe's Faust, Mephisto and God make a bet; if Mephisto can claim the soul of Ariel, God's favorite man, then he will be allowed to reenter Heaven. But if Mephisto fails, he must be condemned to Hell forever. Musically, we are given scattered glimpses of the events to come through distorted fragments of lyrics, accompanied by an erratic strings ensemble.
 Center of the Universe
We are introduced to Ariel's "always searching" mind. He guesses that, at some point, on the verge of death, someone must have known the truth he seeks. Without that truth, he feels alone in his homeland--the center of his universe. A moment of calm enters Ariel's raging mind (represented musically by a melodic duet) in which he decides that, having exhausted every venue of knowledge in his homeland, the answers must lie somewhere else.
Ariel bids farewell to his old life. He burns all bridges and cuts ties with everyone he has ever known--including Helena. Finally, he rejects God, who is apparently unable to remedy the questions and afflictions of the human condition. Eager to begin his quest, he boards the next available ship and sets sail across the ocean, never to return.
 Interlude I - Opiate Soul
This interlude marks the passage of time as Ariel spends many years traversing the wide world in search of truth.
 The Edge of Paradise
Grasping at straws after years of fruitless travels, Ariel has turned to the occult and hallucinogenic drugs (represented through slow, meandering vocals and an exotic Arabesque melody), to which he has developed an addiction. Under the influence of these practices, he experiences a brief vision of Mephisto, though, as of yet, he knows nothing of the fallen angel or his divine bet. Ariel realizes that his current lifestyle will not lead him to the great answer, which he still desires as much as ever.
In despair, Ariel gives up his search and wanders, alone and aimless, throughout the wilderness of winter (the season in which the rest of this tale will take place). He reminisces about Helena, longing to return once again to the youth they spent together.
 Interlude II - Omen
This slow, soft instrumental interlude marks Ariel's darkest hour, in which he prepares to take his own life. But Mephisto, his bet not yet won, has other plans.
 Descent of the Archangel
Bathed in a halo of moonlight, Mephisto descends in angelic glory to Ariel. This is represented musically through a haunting saxophone opening pierced with simplistic keyboard chords. He introduces himself to Ariel, and proposes a deal: He will serve Ariel in life, fulfill all his desires, and grant him all manner of worldly power--on the condition that, when Ariel dies, Mephisto keeps his soul. Though the offer is tempting, Ariel is initially wary.
 Interlude III - At the Banquet
Ariel suddenly finds himself transported to Mephisto's glorious castle, where the fallen angel is holding a feast in his honor. The revelrous mood is set through the upbeat tempo as Mephisto makes his entry to the banquet hall.
 A Feast for the Vain
Ariel enjoys a night of gluttony, revelry, and pleasurable company at Mephisto's hand. Awestruck and overwhelmed by the splendor and glory (shown musically through almost Waltz-like progressions), the last of his doubts are banished; he accepts Mephisto's friendship and consents to his deal--with one modification: If Ariel ever experiences a moment in which he is so content that he wishes to linger there forever, only then will Mephisto keep his soul. Mephisto grudgingly accepts this modified contract.
 On the Coldest Winter Night
Leaving Mephisto's castle, Ariel runs into, of all people, Helena, who, unbeknownst to him, has been searching for him these past years. A moving acoustic bass portrays the warmth and love that still remains between them, even after all their years apart. The two lovers hold each other close and spend the winter night together in the nearby Town. During this night, Helena conceives Ariel's child--though neither is aware of this.
 Lost & Damned
After tarrying in the Town with Helena for a time, Ariel decides that, with the newfound power Mephisto has brought him, it is time to continue on his quest. But he is unwilling to subject the pure and good Helena to the hardships of his journey and the seductive ways of Mephisto. And though he loves Helena, he holds his sought-after answers more valuable than the bond they share. In his words, "Love means nothing to me, if there is a higher place to be." So, though he still loves Helena and Helena alone, he leaves her. The discord between the two is portrayed by a fast-paced drum beat punctuated by slower keyboard chords.
 Helena's Theme
Distraught, Helena vows that she will love Ariel even in death, and drowns herself--and, by extension, the child in her womb--in the nearby River. At the prayer of the River Spirit, both Helena and the unborn child ("Alena") ascend to Heaven.
 Interlude IV - Dawn
As dawn rises, the Town Crier announces Helena's actions of the previous night. Her body has been found in the River, and discovered to be with the child.
 The Mourning After (Carry On)
In a dream, Ariel witnesses the twofold death. His dread is confirmed by the Town Crier. He is shocked and bewildered both by Helena's death, and the death of the child, of whom he had no knowledge. Overwhelmed with loss and loneliness, he grieves for Helena, yet he knows that he must carry on in his quest. After all, if he fails, then what did she die for? It would be a dishonor to her memory not to fulfill the task for which he left her, and compelled her to take her own life.
 III Ways to Epica
Mephisto encourages Ariel to continue his quest, rationalizing Helena's death as a cost of "playing with fire." He asks for Ariel's trust and berates him for his grief, proclaiming all human emotions, which are born in passion, as ultimately destructive. Helena, now in Heaven, declares that love is the only thing that humans can truly know, and that, against it, the forces of darkness can never prevail. Ariel, in despair, blames God for his sorrow. His guilt for the part he played in Helena's death consumes him, and he feels that he is beyond redemption.
The Black Halo plot:
March of Mephisto
In Ariel's moment of weakness, Mephisto's hold on the alchemist's will is at its highest point. He finds Marguerite, a beautiful girl in the Town. She looks and sounds like Helena. In an attempt to console Ariel, Mephisto delivers her to him, paving the way for her seduction. His control and power over Ariel are represented musically by harsh bass lines and death grunts. He convinces Ariel to follow through with this course of action. Thus, as Marguerite seduces Ariel's body, Mephisto seduces Ariel's mind.
 When the Lights Are Down
Both seductions are completed. Mephisto gains control over Ariel's will, and Ariel's illusioned lust for Marguerite is consummated. The two sleep together. Afterward, Ariel's mind emerges from Mephisto's grip, and he realizes who he has slept with. He yearns for Helena, and, as his mind begins to recover, remembers her death, and the events leading up to it.
 The Haunting (Somewhere in Time)
Ariel explains himself to Marguerite as best he can. He explains that he saw her as a mere shadow of his dead lover, that any feelings he has for her are a mere facade of his love for Helena, and that he can never love her the way she loves him. In a tragic and moving duet, he urges her to leave and forget about him. But he says that, perhaps, they may one day reunite. With that, he leaves, never to see her again.
 Soul Society
We once again enter Ariel's troubled mind. He laments the afflictions with which humanity is chained. He wonders how such evil and misfortune came about as the result of his good intentions of finding the ultimate truth. He remarks on the insignificance of human life. Above all, he wishes that humans could live in a free and perfect world, but knows that it will never happen. And once again, he is tormented by his role in the death of Helena and the unborn child.
 Interlude I: Dei Gratia
This interlude marks Ariel's realization that he can never find the great answers in this universe, and that the ultimate truth must lie in Heaven alone.
With this, Ariel is struck by a sudden realization: Because of the sins he has committed and the binding promise he has made with Mephisto, he has severed any possibility of connecting with the divine. He can never enter Heaven, never see Helena again, and never find the ultimate truth. For the first time, he realizes that he is truly all alone. Musically, this begins with a very soft ballad form, gradually building into a "power ballad." Lost and confused, he calls out in despair to God. In a daze, he leaves Mephisto, wanders out of Town, and crosses the still frozen River.
 This Pain
In his mind's eye, Ariel looks back on his journey. He reflects on leaving Helena and her resulting death, and his abandonment of Marguerite. He has forsaken the two women who loved him. He realizes that he is the cause of this sorrow and pain, and that he will never be free from the consequences of his actions.
Ariel decides that he cannot possibly repent for all he has done, and that even if he could, mere repentance would not be enough. This prompts him out of his reverie, into action. Setting out under the moonlight, he sails back across the now melted River, in search of Mephisto. Once across, he approaches the demon's castle.
 Interlude II: Un Assassinio Molto Silenzioso
Standing outside the castle, Ariel looks at the full moon, wondering if anything besides death and damnation lie in his future.
 The Black Halo
A defiant Ariel resigns himself to death. He denounces Mephisto as a liar and traitor, and challenges him. He no longer fears the eternal darkness that inevitably awaits. He resolves to live a life of purity and goodness like Helena did, even though he knows it is too late for him to escape damnation. He embraces righteousness, not for any reward or redemption—there is no hope of that—but simply to be righteous.
 Nothing Ever Dies
Ariel reflects that every human, throughout all of history, struggles with these same questions of right and wrong, love and lust, righteousness and sin, faith and doctrine. In a sudden moment of clarity, he comes to a profound realization: Love, including the love between Helena and himself, is the only universal truth. In this revelation, he realizes that he not only knew this truth all along, but that, for his part, he created it.
 Memento Mori
This realization prompts Ariel to enter that moment of supreme contentment, spurring his deal with Mephisto into action. Musically, it begins almost like a ballad, but quickly builds in speed and intensity. In this moment, as Ariel's soul leaves his body, everything falls into place within his mind. He understands, at last, that, even having found this universal truth of love, he can never be truly satisfied in this world; that death comes to everyone; that no human is simply good or evil; that he alone is the master of his destiny; and that he creates the meaning of his own life. With this final existential revelation, under Mephisto's binding promise, Ariel's soul departs. But it does not come into Mephisto's clutches. Having rejected evil even in the face of certain damnation, Ariel has, beyond all hope, redeemed himself at last. His soul is saved, and ascends to Heaven to join Helena. Mephisto, his divine bet lost, is cast forever into Hell. Musically, this is shown through his long, wailing death grunt and screams. The song fades back into the ballad form from which it began. Here, the story of Ariel properly ends, but there is a bit more to the tale.
 Interlude III: Midnight - Twelve Tolls for a New Day
In an inversion of the "Prologue in the Theatre" from Goethe's Faust, we see that this tale is in fact a play put on for the New Year's festival in the town of Gatesville.
An uplifting, medium- and fast-paced tribute to comedy and tragedy, life and death, joy and sorrow. The overall message of renewal is personified in the penultimate verse: "What does the winter bring, if not yet another spring?"