Bart the General

Their friend, Nelson steals Lisa's cupcakes. Bart tries to retrieve it by force and fails. Then Bart combines forces with the other children, who have been bullied by Nelson, and they force him to apologize.

Bart the General (http://simpsons.wikia.com/wiki/Bart_the_General)

A friend of Nelson's steals Lisa's cupcakes. Bart tries to force him to give them back to her but fails. Then Bart joins forces with other children who are also fed up with Nelson (or have been bullied by Nelson), and they force him to apologize.

Here are some notes about my corrections, hoping that they can be of help.

1. The Nelson friend: in English, you use the definite article "the" when you are referring to something you have already talked about before. In this case, you haven't introduced yet one specific Nelson friend which you are now referring to.
See: http://blog.writeathome.com/index.php/2012/02/getting-articles-straight-a-an-or-the/

2. You can say: "A friend of Nelson's" or "A friend of Nelson". The first form is "more correct" and sounds more formal. The second is also considered correct and is more informal.
See:
- the song "Still a friend of mine" (note that its name isn't: "still the my friend"): https://www.youtube.com/watch?v=os6IB1MdSBU
- a relating question on a forum: http://forum.thefreedictionary.com/postst74566_I-m-a-friend-of-John-or-I-m-a-friend-of-John-s-.aspx

3. cupcakes of Lisa -> Lisa's cupcakes  (use of possessive case)
See: http://www.learnenglish.de/grammar/casepossgen.html

4. I think that the expression "to combine with" is used with other meanings.
See: http://dictionary.cambridge.org/it/dizionario/inglese/combine

5. affected: if you are affected by something, it means that something has some kind of effect on you, but it doesn't necessarily mean that the effect is negative
See: http://simpsons.wikia.com/wiki/Bart_the_General