This is an idea from a game I played, Ayakashi
Basically there is an Agency, the Agency of all Supernatural Activity. These agents focus on calming down Spirits that roam the city. Only people able to see spirits can calm them, normal people cannot see, hear or touch them, in comparison to people who have the gift. Once calmed a spirit can choose to pass on or serve the person that calmed it, but is then called a deamon.
There are three types of spirits/deamons;
They are described as ghosts with a strong embodiment to hate or to feel the need to inflict malice and havoc.
This class tends to include creatures and characters like witches, demons, and lesser divinities.
Phantom types, in battle, have an advantage over Anima types, but tend to be weaker against Divina types.
They are described as embodiments of objects and weapons. These include modern weaponry like guns, tanks and planes to swords, kaleidoscopes, and legendary items.
Anima spirits are most likely Tsukumogami, abandoned objects that have been forgotten that come alive after the course of 100 years. Objects can also become Tsukumogami if they have strong feelings towards their master (love or hate).
Anima type, in battle, have an advantage over Divina types but are weak against Phantom cards.
This class includes gods and hero-type characters. In battle, Divina types have an advantage over Phantom types and a disadvantage against Anima types.
Every deamon has a certain rarity, 5 stars being the highest and 1 being the lowest. The deamon itself is fitting to that rarity, but can evolve into a higher rarity at a higher level.
Starting out as a 1 star rarity card you will evolve to a higher rarity at level 40, 50, 60, 70 reaching rank 5.
Now for the actual plot:
Lately bountyhunters have been popping up more and more in the city. This is most likely caused by the mass amount of Evil Portals that have been opening around the town. An Evil portal spreads negative energy and affects the spirit in the area, causing them to run havoc. When a spirit loses it's mind normal people can see it and feel it's attacks, which is what the agency is trying to stop from happening.
You are an agent/civilian/bountyhunter able to see spirits and are trying to find out what causes the mass amount of evil portals and if you can stop it/capture and sell it.
So, basically thats it ^^"