Go into the fifth floor of the Silph Co. building in Saffron City. Go to the bottom of the screen to find a man standing next to a teleporter. Enter the teleporter to travel to another room. Get off the teleporter, then go in it again to return to the fifth floor. Go right to find a ball that looks like a Pokeball.. Pick it up -- it should be a card key. Go back in the teleporter you used before. Get off the teleporter, then enter it again. Go up and go to the third floor. Go down, then go left to find a "wall" blocking the way. Talk to the "wall" and the card key will open it. Go left to find a teleporter. Enter the teleporter, then go left and you will battle Gary. Talk to the man near Gary and he will give you a Lapras. Keep going down, then four steps right. Go up, then talk to the "wall" to open it. Go up and battle Giovanni. Defeat him, then talk to the man (Mr. Fuji). He will take you to his house and give you a Master Ball. This Poke Ball will catch Pokemon 100% of the time. You can only use it once, so use it on rare Pokemon such Moltres, Mewto, Zapdos and Arcticuno.