When we left off Goku had agreed to help Vegeta fight Frieza on planet Namek. I also stated that Goku and Vegeta got a boost from traveling to Namek. Bulma begins modifying Nappa's pod to hold Goku (2 days). She learns that while in their pods Saiyans have controlled dreams about their most epic fights a third of the time they are travelling to prevent them from going soft while sitting still for so long. Knowing that Goku is still behind Vegeta she intensifies his too about 3/4 of the time. Vegeta doesn't want her tampering with his pod but dose show interest in her. (slightly and casually as it is his tough guy attitude that draws Bulma to him)
Ok so on to the Yamcha. Goku ask Yamcha to meet him before they depart. Goku states that he never really wanted to be earth's guardian but protecting his friends and training to fight is what he's good at. He ask Yamcha to look after Gohan and protect everyone now that Earth's real guardian is gone Kami and he is leaving. Yamcha agrees glad to be but sayes he needs some of the special training Goku and Tien had. Goku sayes and assumes Roshi and Shen will train him. The Masters agree to train him but both feel right away that they shouldn't teach him the Kaio-ken. They regret to some existent teaching it to Goku and Tien. Because they know it is too dangerous. Goku and Vegeta leave. Yamcha begins his training and dose well raising his power level and skills from 900 a bit above where he was when the Raditz saga takes place, to 1,900.
Dr. Gero's spy bots have told him that Goku has left and Piccoolo, Tien, Krillin and Chiaotzu are dead so he decides that now is the prefect time to strike. However he knows that he has under estimated the Z fighters before so he builds an android with one power absorption hand (something at the time he wasn't sure could be done). He decides that a power level of 5,000 should ensure his victory (he dosen't know about Vegeta and Nappa yet or Kaio-ken) and that if this prototype Android is successful it's time to make himself immortal as well.
Obviously when the Android strikes Yamcha more than has his hands full. Luckily the Wolf Fang Fist confuses the Android's defenses and can't be absorbed but with such a large power difference the Android doesn't need to absorb any power and can tank Yamcha's attacks. Yamcha puts up a much better fight than he should (much like Tien in the real DBZ against Cell) but also like that fight he dose almost no real damage and barely manages to hold the Android back. Yamcha pushes the fight away from the city trying to save as many people as he can. It turns out Nappa has been getting his peacefulness on in the woods since the fight with Tien. Nappa lost his memory because the fight really did almost kill him also. When a Ki blast kills Nappa's new friend a wee little birdy Nappa goes berserker on the Andriod. Yamcha still having the quick thinking of his thieving days tell Nappa that he is Yamcha's bodyguard. And that he's been lost. Nappa reluctantly excepts this new role as he did seem to be quite skilled at kicking that other guy's a$$. And they begin training together. (Yamcha dosen't know who Nappa really is But hopes that training with him will bring his power level above even Goku)