If water is coming through the ceiling material, it usually means the roof is leaking, and it should be considered an emergency. Left alone, water can do a great deal of damage inside a house.
If an area of the plaster or Sheetrock® is stained or bowed, this should be cut out as soon as possible. You don't want water-saturated ceiling material coming down on someone's head.
The next step is to find the leak, which is easier said than done. Many times the water enters at one point, and then flows down a rafter or something else to emerge elsewhere.
If it is raining, and the attic is open, poke your head up there and shine the light around. If you can see where the leak is coming from, mark it with crayon or the like; or if a hole is visible, stick a wire through to the outside so you can observe it and know where to make the repair when the weather is right.