Place a large pot of salted water over high heat and bring to a boil. In a large mixing bowl, whisk together all of the ingredients for the dressing; reserve.
Drop the soba noodles into the boiling water; once the water returns to a boil, reduce the heat to low and simmer the noodles until tender, 2 to 3 minutes. Drain the cooked soba noodles and run them under cold running water to chill them thoroughly. Transfer the noodles to the mixing bowl with the prepared dressing.
Add the scallions, basil, kale and plums to the bowl and gently toss to combine. Adjust the seasoning as needed with tamari or soy sauce and Sriracha. Serve chilled or at room temperature.
If you're craving a lighter meal, just enjoy the noodles as they are. If you want a bit something more, they lend themselves perfectly to accompaniment by some seared salmon or grilled chicken or pork.