Dragon Quest V starts off with your hero's birth. Obviously you can't play as a newborn infant so after a cut-scene, you'll flash forward 6 years. For some reason yet unknown to the player, your father, Pankraz, is no longer king (your hero doesn't know that he ever was king). He now travels the world looking for something and you travel along with him. Sounds like a pretty sweet deal to me. At this point you'll learn how to fight, meet a new friend or two, and help some people out. As long as your dad is with you, you won't be able to die. He will heal you after every battle. You can also use his midheal to heal your party members. He never runs out of magic either.
Unfortunately for your hero, life doesn't stay this great. Now I'm not going to spoil the plot by telling you anything about what happens. What I will tell you is that you will play as the hero in three different stages of his life: childhood, teenage years, and adulthood. It's quite nice to see how your character grows up in these different stages.
There are a few things I should tell you about the game here. First, especially at the beginning of the game, your hero will be the one consistent member of the party. So personally I wouldn't waste any seed that you might find on your other players. In the first stage of his life especially, the game is quite easy so I didn't see the need to give a temporary character benefits that your hero would be able to use throughout the game. Maybe that's just me though. Second, don't worry about losing all your stuff between the first and second stages. You will get it back eventually.
Third, you will be able to recruit monsters to help you starting in the second stage after you get a wagon. These monsters will be quite helpful after you level them a bit. They will not necessarily listen to your commands until their wisdom is at least 20, so don't count on them to do what you told them to do in battle! I would recommend not selling back your weapons even from the beginning unless you're dying for the money. That way, you won't have to buy as much for your monsters. Different monsters require different equipment, so I kept everything until I was sure I didn't need it again.