Plaster was a material used in many older homes before the use of drywall. The wooden parts of the walls were covered in plaster. A plywood sheet covered the wall studs, and plaster was used to cover that. When drywall was invented, plaster was still used to cover the drywall to make a more even finish. When there is a problem with damp plaster, whether from water damage or a leak somewhere, it is necessary to fix it or the whole wall could deteriorate. However, there is no feasible way to fix damp plaster. The best thing to do is remove the old damp plaster and start over with new.
Use a can opener or bottle opener to remove the damp plaster. Undercut the damp area a little so that you have removed some of the dry plaster as well. If there is an issue with a leak or wet wood, replace that before continuing. Any repairs you make to a wet surface will only mess up again.