Preheat the oven to 400°F (200°C).
Score the skin of the duck breast in a crisscross pattern, being careful not to cut into the flesh.
Season the duck breast with salt and black pepper.
Heat olive oil in an ovenproof skillet over medium-high heat. Place the duck breast skin-side down and sear until the skin is golden and crispy, about 5-7 minutes.
Flip the duck breast and sear the flesh side for 2 minutes, then transfer the skillet to the oven.
Roast in the oven for about 8-10 minutes for medium-rare.
While the duck is roasting, prepare the plum sauce. In a small saucepan, combine plums, honey, balsamic vinegar, and soy sauce. Simmer over low heat, stirring occasionally, until the plums are soft and the sauce has thickened, about 10 minutes.
Remove the duck from the oven and let it rest for 5 minutes before slicing.
Serve sliced duck breast with plum sauce drizzled over it.