You can obtain HM05 Flash by talking to Professor Oak’s aide located in a gate on Route 2. This area can be accessed two ways:
- Going through Diglett’s Cave (east of Vermilion City).
- Or going north from Viridian City.
The second path is much shorter, but which one is more convenient depends where you are in the game.
For example, if you’re already in Vermilion City or its surrounding areas, it would be a hassle to have to backtrack to Viridian City without Fly. So generally speaking, Path 1 through Diglett’s Cave is usually your best choice.
As for the move itself: Flash can be used to light up Rock Tunnel, which is the dark cave that leads to Lavender Town.
But it isn’t required since you can still move around inside the cave without Flash. You can even beat the game without ever needing to use Flash at all!
Still, it is a helpful HM that could save you time and energy when navigating Rock Tunnel.
Regardless of which way you take, you will need the following before you can obtain HM05 Flash:
- A Pokémon that knows HM01 Cut in your party
- At least 10 Pokémon registered as “owned” in your PokéDex
If you don’t have HM01 yet then you can follow our guide here to get that HM first.
Once you have that, you should be able to reach the gate on Route 2. But the aide will not give you HM05 until you’ve caught (or evolved) 10 total unique Pokémon.
Note that you have to capture 10 different species of Pokémon to fulfill this requirement – just seeing 10 different Pokémon isn’t enough.
Path 1: From Vermilion City to Diglett’s Cave
Go to Vermilion City and take its southeast exit.
You’ll know you’re headed the right way if your path has you go east past the PokéMart and the Vermilion Harbor.
This will take you directly to Route 11, and Diglett’s Cave should be immediately visible from when you enter.
Head into the cave and go down the ladder beside an old man. This leads to the main section of the cave.
It’s a long path, but it’s straightforward and has no branching points. Continue walking along it until you reach another ladder at the very end.
That ladder should lead you to a small chamber with a Hiker where you can find the exit.
Upon exiting the cave, you will be entering Route 2.
Head further down and you should find a house and some fencing blocking the rest of the area just below it.
Between the fencing is a small tree you can remove, so have your Pokémon use Cut on it and continue making your way down.
The gate where you’ll find Professor Oak’s aide is just past two ledges from here.
Skip to the final section of the guide if you have trouble finding the aide.
Path 2: North of Viridian City
Starting from Viridian City, head north to Route 2. This is the same way you would take if you were going to Viridian Forest.
Keep going up until you see a small tree to the right.
Use Cut to get the tree out of your way, then go up to the new area you’ve opened.
Keep going until you find yourself in the gate where the aide is staying in.
Finding Professor Oak’s Aide on Route 2
He isn’t difficult to find since there are only two people inside the gate. He’s the one with brown hair, glasses, and a white coat.
If you have caught at least 10 different Pokémon when you speak to him, then he will give you HM05 Flash.