In the forest of Route 8, just east of Vinoville Town, lies a white house where a woman named Ripley lives. She is having trouble with mysterious crop circles appearing in her fields and wants you to investigate possible alien sightings.
To begin, you need to visit Ripley at night and agree to look into the matter. You will then be transported to a closed-off section of Route 8, which consists of wheat fields being patrolled by a number of S51, some of which are openly moving about while others remain hidden in the wheat. There are no wild Pokémon, Trainers, or items to be found in this area.
Make your way through the fields to reach the lake shore without being spotted by the S51. If they spot you, you are brought back to the starting point and must start again. Upon reaching the lake, a LVL 35 S51-A will see you and attack. You cannot capture this S51-A.
After the battle Ripley will approach, realize the S51-A was directing the S51, and wonder why it was doing so. Sheldon will appear (though Ripley doesn't immediately recognize him) and condescendingly mocks Ripley and Vinoville Town for being so boring. He'll acknowledge the player's skill in defeating his S51-A and say he looks forward to their upcoming Gym battle. Jumping on his S51-A, he leaves, and Ripley tries to explain his actions. She'll comment on the rarity of the alien Pokémon, and not feeling confident in her skills as a Trainer, she will give you an S51.
This is the only way you can catch one in the game. Prior to 1.0.4, you cannot come back for it after saying "No". After this update, after saying "No" the first time, you may asks Ripley for the S51 a second time and after saying "No" the second time, S51 cannot be retrieved.
Ripley is an obvious nod toward the sci-fi heroine Ellen Ripley, who had a run in with extraterrestrials of a bit less amicable nature in Alien.
In previous versions saving during Ripley's Sidequest would cause the game to crash. This was fixed in Patch 1.0.4