When civil engineer Cael Ward dies in a construction accident at age 30, the last thing he expects is to wake up in the body of Sarek Ashford—the useless, debt-ridden third son of a minor noble family in the Kingdom of Morcelon. According to the novel he read before dying, Sarek is destined to drag his family into bankruptcy within two years, leading to their execution when they can't pay taxes to the tyrannical Duke Alde.

Armed with modern engineering knowledge and a desperate need to survive, Cael starts implementing infrastructure projects to save the failing Ashford estate: aqueducts, proper sewage systems, crop rotation, and revolutionary construction techniques. What should be impossible for a medieval world becomes reality under his guidance.

Enter Lord Ryn Alde.

The Duke's younger brother and the kingdom's Commander of the Knights is everything Cael expected from the novel's descriptions: devastatingly handsome, politically brilliant, a master swordsman, and apparently sent to inspect the sudden transformation of the worthless Ashford estate. In the original story, Ryn was a minor character—the cold, duty-bound knight who eventually signed the Ashford family's execution warrant.

But the novel never mentioned how Ryn's icy composure would crack when Cael explained the Pythagorean theorem. Or how he'd start visiting the estate weekly to "supervise construction projects." Or how his steel-gray eyes would soften when Cael got too enthusiastic about architectural plans.

As Cael revolutionizes the kingdom through infrastructure (and accidentally becomes famous as "The Builder Lord"), Ryn finds himself drawn to the strange lord who speaks of things that shouldn't exist, builds wonders with mathematics, and looks at him like he's a person rather than a weapon. What starts as suspicion becomes fascination, fascination becomes friendship, and friendship becomes something far more dangerous.

Between managing construction projects, navigating noble politics, preventing the kingdom's prophesied destruction, and trying not to fall in love with the man who was supposed to execute his family, Cael realizes that building infrastructure is easy. Building a relationship with a medieval knight while hiding his transmigration? That's the real engineering challenge.

Especially when Ryn starts looking at him like he wants to be blueprinted too.