The CAT can't do anything to the spark plugs - it's downstream in the exhaust. Chances are the spark plug was damaged first. If the CAT is blocked it'll choke up the engine with exhaust fumes that can't escape and that will cause it problems but it won't go forever like that without doing something stupid, either blowing open the EGR tube, or the CAT.
Did you have a long range fuel tank added? Did you refuel at an old service station, or one that isn't busy (low turnover)?
Dirty fuel will foul the spark plugs and could cause damage to the CAT by filling it with debris. This may also cause injector damage. Petrol injectors may not be under the immense pressures that you'll get in a comon rail diesel, but they can still suffer from water droplets or debris.
Driving through a serious water puddle and getting water into the airbox may also cause some of that water to reach the engine, and both petrol and diesel engines don't fare well with a lungful of water. Checking the air filter for water damage would be a good step if you've driven through deep puddles or lent the car to someone who might have. Water in the intake manifold can cause problems with spark plugs.
How many km has the car done?