Phineas Gage is a strong and hard working miner, he and his partner are completing there routine of filing up there holes that will open up the tunnel. Except that his partner didn't cover a hole with sand like he usually does.
Phineas's iron rod dropped in the hole without sand and set off the gunpowder explosion. The iron rod goes through his check and the frontal lobe of the brain, which controls the emotional part of the brain, and out his skull but he is still alive.
Those who know him they know that he is only physically healed and that he never will be mentally healed. Him not being mentally healed means that he will have random temper tantrums over nothing and sometimes he will be cheerful.