This adventure I started a while before. But it had only two levels, I added now 2 levels. The interactions are like:

			collx = Mathf.Abs(ego.transform.position.x - kaibaman.transform.position.x);
            collz = Mathf.Abs(ego.transform.position.z - kaibaman.transform.position.z);
            if (collx < 2 && collz < 2)
            {
                anzeige.SetActive(true);
                if (inv < 3) info.text = "Here is such a nice arena with audience. I wanna fight. But I don't have a weapon!";
                if (inv == 2) action.text = "g)ive him the sword";
                if (Input.GetKey("g") && inv == 2)
                {
                    inv = 3;
                }
                if (inv > 2)
                {
                    info.text = "Thank you! I give you for this my magic shield. It can tell you tales.";
                }
            }
			
ego is the First-Person-Controller, the player. The other gameobject is the NPC. It's quiet simple. You as the player
can carry only one item and you must bring it to a special person.