Sand off the paint from the seam with a belt sander and 80-grit paper. Sand about 6 inches on either side of the seam. Remove all the paint until you reach drywall and joint compound.
Reload your belt sander with a fresh belt of 80-grit sandpaper (the paint will have clogged up the previous belt). Run the sander over the seam to remove the plaster. When you get down to the connector that was used between the sheets of drywall (either mesh tape, or paper tape) pull it off. Make the surface smooth and free of loose plaster.
Lay a strip of mesh tape over the exposed seam. Use your 6-inch drywall knife to spread a thin line of joint compound over the tape, just covering it. Let it dry for a few hours.
Sand down the seam with your drywall hand-sander and screen. Use your 12-inch knife to spread a wider line of compound over the seam, about 6 or 8 inches wide. Taper the line of compound at the edges. Let it dry.
Sand the second coat of compound with your hand-sander, and apply a third coat, using your 12-inch knife. Make the seam about 10 inches wide, tapering it at the edges. Once it dries, sand it very light to get the seam completely smooth.
Repaint the repaired section of the wall.