Underneath this there will be brick.
The dot and dab on a damp wall can force the damp through the dabs and you see it through the plasterboard.
If it were me I'd remove any loose plaster, try and sort out any signs of damp, then use bonding coat to fill it up to level and skim the wall.
How brutal would you be on the loose plaster front? And what about damp plaster? There's some that feels like wet clay - I wipe it with my fingers and look like I've been abusing Morph.
We had some dodgy spots and went for a start again approach. Didn't DIY though.
@CYOA started
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Underneath this there will be brick.
The dot and dab on a damp wall can force the damp through the dabs and you see it through the plasterboard.
If it were me I'd remove any loose plaster, try and sort out any signs of damp, then use bonding coat to fill it up to level and skim the wall.