Crack in plaster/ wall?

How wide is the crack? Does it run diagonally, or vertical?

I won't go that far, but it's true that plaster walls can be difficult to repair--especially if they are too far gone. Like rust on a car, you need to strike at the first sign of problems. You'll want to preserve whatever is there and fix it, rather than tearing it out. The moment you start tearing out chunks of plaster, it's a never-ending process. One chunk leads to another, and before you know it, your backyard is full of plaster and you're shopping for drywall. 1.Using a putty knife or a utilty knife, score the hairline crack to open its edges. This may seem counter-intuitive, but you need to increase the area for the repair compound to stick to. Brush off any loose crumbs. 2.With a wide taping knife (of the type used for drywall), smooth a thin layer of joint compound over the cracked section. 3.Press paper tape or fiberglass tape into the wet area, along the length of the crack. This is ordinary drywall tape. You don't need to purchase any special plaster repair tape. 4.Let dry. Feather joint compound over taped area so that the compound extends two or three inches past the taped area. 5.Let the joint compound dry. Lightly sand it down with fine sandpaper to get rid of any bumps or ridges. Don't sand so hard that you dig into the tape. 6.Feather a second layer of joint compound, this time extending the edges even farther to about six to eight inches. Let dry. Sand. 7.Finally, your third coat of joint compound takes the edges out to twelve inches. Since this is your last chance to get the sanding right, be careful to make it smooth.

Hello. The crack is most likely caused by sinking foundation. The depth of your foundation might not be sufficient for the type of soil it is built on. Underpinning is often a good solution. However it is hard to tell provisionally as it must be seen first. Regards, Pavel