Derrick explains that his mother went into denial after her other son's death. As a result, she never spoke about him and she discarded anything that reminded her of him. Derrick had trouble adjusting to this and slowly started to forget even what his brother looked like. His only tie to him was one photograph.
It is soon learned that Derrick is hearing a voice which he believes is his late brother. Park tries to convicne Jen to help Derrick deal with his grief, but she refuses.
Derrick is finally diagnosed with a persistent stapedial artery which, when corrected, will put an end to the voices and his other symptoms. However, when he feels he will lose his brother, he attempts suicide. When Gregory House learns of this, and largely due to his difficulty dealing with Wilson's decision not to treat his thymoma, he physically attacks Derrick while Jen tries to fight him off and explain that Derrick had a momentary lapse of judgment. House goes on about how Derrick's fighting back shows that this is the normal reaction, but Park has to hit him over the head to get him to stop.
After Derrick is successfully treated, Jen brings more pictures of his brother for them to look at.