Many homes, especially beautiful older vintage homes, have plaster walls in them instead of the more modern sheetrock walls. But to fix a damaged plaster wall requires skills, tools, and procedures that are not at all like those used to repair other kinds of walls, so to do so you will want to employ a special plaster wall contractor.
First the old plaster surface needs to be cleared away, and behind this plaster coating there will be a subsurface that is typically made of chicken wire and wooden slats. Depending on how bad the damage is, the plastering contractor may be able to just mix up a new batch of plaster and apply it in careful layers to fix the damage. But if the substrata is also damaged -- for example if the chicken wire is broken or bent or the wooden slats are broken or rotted -- then it has to be repaired first.
Once the wooden surface is secured and prepared the plastering contractor will nail or screw wire mesh or chicken wire to it to create a porous surface that the wet plaster will stick to nicely. Then your plaster wall contractor can begin troweling on fresh plaster, layer by layer, to build up a nice smooth surface of plaster that will later dry to become your new and improved plaster wall. Done right the plaster repair may last for 100 years or more.
As any plastering contractor can explain to you, plastering work is an old-fashioned craft and trade that is seldom done these days, so your plaster wall contractor will be invaluable to you if you live in an old house with plaster walls. Eventually every home needs a repair, and your plastering contractor will know how to get the job done correctly so it looks beautiful and works like a champ.