Return To Route 2: Mr.Mime & HM Flash
Description:
Once you make it through Diglett Cave, you'll come out on the opposite side of Route 2 that you didn't have access to earlier in the game. There still isn't much on this route, but do get the chance to Trade for another Pokemon and obtain HM05 Flash.
In the building right outside the exit of Diglett Cave is a trainer wanting to Trade their Mr. Mime for an Clefairy. Clefairy is pretty difficult to find and catch, but Mr. Mime is impossible to find anywhere else in the game. This trade is a must at some point.
In the larger building south of the Diglett Cave Exit is one of Professor Oak's Aides. He is there to give you HM05 (Flash), but only if you can caught ten different kinds of Pokemon. You need this HM, mainly because of Rock Tunnel. coming up in the game.