Use The Skip Mission :)
SimPle
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You must: 1. Beat the 6th gym leader. 2. Go to Twist Mountain 3. Before you go in, Clay, the 5th gym leader, will give you surf.
first you go get 4 other gym badges and go to petalburg city and beat your dad Norman and go to the house beside it where wally lives and talk to the mom or the dad i don't remember then 1 of them will give you the move surf
Well after you beat it the second cave go to cerulean city go through the bridge in the north go left and down till u can surf. surf down and left when you can and there will be someone blocking it. If ou beat him 1 time it wont work. if second he should let you in. then in some place that is there should be Mewtwo. Well ive only telled you how to get mewtwo. well the answer to your question you unlock mewtwo.
Gardevoir lvl 33 Psychic, Thunderbolt,Imprison, Calm Mind Tentacruel lvl 31 Surf, Ice Beam, Supersonic, Acid 1) Use imprison so solrock and lunatone cant use psychic or calm mind 2) use surf 3) keep using calm minds for gardevoir 4) keep using surf for tentacruel 5) destroy them 6) remember to use healing items
The only way I know of to get it is from the Battle Subway for 1 BP.