Shut the water off before anything else
Nothing else matters while the water is still running. Close the main shut-off (see our guide on finding it in a triplex), then open a low tap in the house to release the pressure left in the pipes.
If water is already touching an outlet, a power bar or the electrical panel, stay away from it. Only switch off the main breaker if the panel itself is dry and reachable without stepping in water.
Limit the damage while you wait
Move what you can, boxes, rugs, furniture, out of the wet area. Towels and buckets slow the water's spread into the rest of the basement, but they never replace shutting the water off.
If water is reaching a wall or ceiling shared with a neighbouring unit, tell that neighbour or the landlord right away: water damage often spreads sideways faster than people expect, especially in a plex where units share the same structure.
Document before you clean up
Take photos and a short video of the affected area before you start mopping up or moving anything, that's what your insurer will ask for first. Note when you noticed the leak and when the water was shut off.
Once the water is off and documented, call the emergency line. The after-hours premium, if it applies, is quoted to you before a plumber is dispatched, not after.