It’s been a while since a serious, period epic of redemption amid political turmoil formed the basis of a large-scale original musical, and for that reason alone this Broadway tryout in Chicago — based on the true story of English slave-trader-cum-abolitionist-pastor John Newton, who penned the words of the beloved hymn “Amazing Grace” — possesses an admirable ambition. That sincere earnestness, along with expert pacing and theatrical flair from director Gabriel Barre and superb, intense performances (particularly from Chuck Cooper and Tom Hewitt), carry this show along with moments of genuine potency. But it’s an open question whether those strengths can ultimately overcome a mostly workmanlike score and a bold but elongated focus on the wretchedness of its main character, whose song-inspiring soul-saving doesn’t occur until halfway through the second act.

No question, by the time the cast sings “Amazing Grace” at the end of a dense but clearly plotted two and a half hours, the uplift has been earned. Starting in the 1740s, the show depicts Newton (Josh Young) as a bratty young man. He returns home to an angry father (Hewitt, giving the most nuanced performance here), the owner of a slave-trading company who had sent his son off to university only to hear that he had instead joined the crew of one of his ships to seek adventure and wealth. Johnny’s idea of being “Truly Alive” — the first and catchiest song — mostly involves drinking, insisting he can handle responsibilities he clearly isn’t ready for, and disrespecting not just his father but Major Gray (Chris Hoch), a royally connected man who has been stationed in town to rid it of potential rebels, and who ultimately forces Johnny into the Navy and sent off against his will.

Johnny’s only redeeming quality is his love for Mary Catlett (in a rich and likable performance from Erin Mackey), who remembers him as a talented and thoughtful boy who used to write her poems and wonders what happened to make him such a jerk. The fact that Mary never stops believing in him is the one element that keeps the audience rooting for his conversion rather than his comeuppance, a structurally smart move from book writers Christopher Smith and Arthur Giron. As if Mary weren’t already over-idealized, she puts herself at risk by spying for the abolitionist movement, allowing herself to be courted by Major Gray in order to help the cause. While the main story is Johnny’s, it’s Mary’s subplot that provides a rooting interest and forward narrative thrust.

Meanwhile, Johnny is busy being a terrible sailor — lashed for lack of discipline — and his ship is attacked by the French when Johnny is gambling rather than keeping watch. He is saved by his slave Thomas (Cooper), who had made a promise to Johnny’s dead mother to protect him and therefore followed him onto the Navy ship. That moment provides an opportunity for highly creative stagecraft, as the ace design team takes us underwater to witness Thomas diving down to rescue Johnny.

The two are then taken captive in Sierra Leone, where an African princess (Harriet D. Foy) carries on her own slave trade. Yes, the white man is made a slave, a bit of biographical plotting that turns out to be historically accurate. (Plenty else that’s here isn’t).

Still, Johnny doesn’t reform but becomes the princess’s accomplice, and, in the single most despicable act among many, sends his savior Thomas off to the dreaded Barbados, where slaves are treated especially horrifically. Afterward Johnny is rescued by his father, who dies in the effort. Filled with apparent guilt now for all he has done wrong, Johnny is about to kill himself when he finds a letter from Mary, and her faith in him, along with the fact of his survival during another storm, finally — finally! — leads to his religious epiphany.

Smith and Giron smartly steer the audience toward accepting Johnny again, as it’s Thomas who grants him forgiveness. Cooper has the two most affecting moments of the show, first when he sings the intense “Nowhere Left to Run” upon being banished to Barbados by the man he saved, and then later when his character forgives Johnny for doing so. The show is really not about God’s grace — which doesn’t get explored deeply in terms of its meaning — as it’s about the fictional Thomas’.

The idealization of certain figures isn’t the most problematic element of the storytelling, though. The musical compromises its theme — of humanity being neither fully saints nor entirely sinners — in the foppish villainy of Major Gray, who isn’t just hateful but laughable. In doing so, they additionally weaken the idealized Mary, who never has to think twice about betraying Gray since he’s so utterly ridiculous. Hoch is juicily detestable as Gray, who, alas, is also granted the clunkiest of all the songs, in which he outlines to Mary his expectations of her.

The pull of the historical and the realistic against the show’s undertow of implied providential miracle makes for a fascinating tension. There’s also the deeply personal and individual sense of God’s presence and mercy competing with the thrust of social justice that drives the larger story. Clearly Smith, a former Pennsylvania policeman who is a newcomer to writing musicals, wants to appeal both to faith-based audiences as well as to more skeptical ones. He and Barre strike a careful balance, but they may be sacrificing depth for breadth of appeal.

The stagecraft and sweep are impressive, but there are opportunities for strengthening the show’s already promising emotional force. Credit Barre with crafting a constant sense of forward momentum, but the production also rushes over some key transitions that might lend even greater uplift at the end.

The bigger issue, however, is Smith’s score, which has some highlights but comes off as capable rather than inspired. Instead the show relies on the audience’s anticipation for the titular hymn — which boasts such a strong brand and such raw power that it may just be enough.