Alan Jones Jordan Farris is a recently unemployed grocery store worker. One night he goes back to find his coworker Ashley Jami Harris and propose to her, but when he sees her kissing his former boss, he leaves.
While heading home, Alan is kidnapped by a child genius named Igor Christian Bellgardt and taken to a warehouse run by Dr. Tanner Finski Thomas Cunningham. There they remove one of Alan’s eyes and graft it into a Frankenstein monster; Igor injects the creature with a special regeneration liquid, and Alan inadvertently triggers a process that brings Frankenstein to life. A bolt of lightning overloads the system, and an interdimensional portal swallows everyone, spitting them out in 1865 in the midst of a brutal Civil War battle.
The Frankensteins rampage across both Confederate and Union lines, leaving a wake of casualties. Alan is rescued by Corporal Solomon Jones Rett Terrell, who discovers that he is Alan’s distant ancestor. Solomon is taken to the infirmary beside the dying Dr. Finski, and Alan begins to glimpse through the original Frankenstein’s eyes. Igor remains behind Confederate lines where Lt. Swanson Lucas Ross retrieves Igor’s injector and allows him to slip over to the Union side. Swanson shows the injector to Capt. Walton Thomas Cunningham, who uses it to inject a cat, watching it transform into a towering beast that races off into the woods.
Dr. Finski warns that the only way back to their own time is to kill Frankenstein, but he dies before they can implement a plan. So Alan, Igor, Solomon, and Virginia Raychelle McDonald—a medic and Solomon’s girlfriend—set out to locate the Frankensteins. They eventually track them to another Union camp. During a skirmish, the original Frankenstein corners Virginia in a hot-air balloon, and they escape together. Virginia tells Frankenstein about her life as a slave and why they fight, and the creature, moved by her story, agrees to help the Union, causing the duplicates to ease their aggression. The balloon is shot down, and they are captured. On the return journey, Solomon is attacked by the cat-beast and loses his left forearm.
Back at the Confederate camp, Walton observes how strong Frankenstein is from the injector, so he orders Swanson to inject himself, transforming into a larger, more formidable Confederate monster. Believing the war is effectively won, Walton lets them go to warn the Union. Virginia reappears at the Union camp with all the Frankensteins, giving the soldiers renewed hope.
The next day a massive, gory battle erupts. With the help of the Frankensteins and Igor’s disintegration ray replacing Solomon’s missing forearm, the Union forces prevail. As the Frankensteins close in on Walton and the Confederate monster, Maggie—Virginia’s sister—stabs the original Frankenstein in revenge for killing her soldier boyfriend earlier. The other Frankensteins suffer, and the Confederate monster disables Solomon’s ray gun, killing Maggie before departing.
When Lincoln is alerted to the victory, he invites Alan, Igor, and Frankenstein to the White House. Igor drinks more regeneration liquid and hands Solomon a bag not to open until he’s gone. In the capital, Lincoln notes that Walton and the Confederate monster have arrived and they must uncover why. They track them to a tavern, where Igor overhears a plan to assassinate Lincoln with John Wilkes Booth Christopher Robinson. Igor is spotted and shot by Walton. As Igor dies, Frankenstein summons the other Frankensteins to regroup, followed by Solomon and Virginia.
Alan and Frankenstein confront Walton, Booth, and the Confederate monster at the theatre. A fierce fight ensues; Solomon and the other Frankensteins arrive, and the Confederate monster is overpowerd. Walton is thrown into the Frankenstein ranks and torn to pieces, while Booth fires at Lincoln but is thwarted when Frankenstein steps in front of Lincoln and is shot. The blow staggers Frankenstein, who topples the balcony, crushing Lincoln. Booth is disintegrated by Solomon as the dust settles. Frankenstein dies, the other Frankensteins vanish, and the portal reopens, allowing Alan to return to his own time.
Back in the warehouse, Alan warns Igor of his impending death, but Igor—believing Alan would die if he stayed—remains committed to leaving. Alan returns to the grocery store, punches his ex-boss, and asks Ashley to marry him. She declines, arguing they’re not ready. When Alan passes Ashley a $5 bill, he sees Frankenstein’s face etched on the note. A quick ancestry search on Solomon Jones reveals that he married Virginia and even adopted Igor, making Igor Alan’s great-great grandfather. The bag Igor gave Solomon contains a metal rod to be placed in Solomon’s grave during a stormy night; a lightning strike activates the regeneration fluid, bringing Solomon back to life.
