- MAY CONTAIN SPOILERS FOR "THE INSPECTOR'S SON"**
Benny is the "son" of the Tenth Inspector. He was created via blood sample on a futuristic planet (Tessamine). Fans have speculated over why the DNA of the Inspector did not carry over to make Benny black, but it's been stated outright since the episode that Infinity Knight DNA remains the same through every metamorphosis.
The Inspector was very disturbed by Benny at first, because of his apathetic nature and unwillingness to fight against the inhabitants of his planet (which they later discover to be 1860's Earth). Mona Virtue also instantly dislikes Benny and begins taunting him with racial slurs, starting a civil war.
During the war, Benny realizes that he's meant to keep the peace between both sides and forces them to stop with a rousing speech, entitled: "I have a dream". One of the fighters, a young man named Jebidiah King, stops to listen and promises Benny he will pass it on to his children. However, in the heat of battle Benny is trampled to death. The Inspector, who has grown to like Benny over time, mourns the loss and leaves with a guilty Mona Virtue. This event is widely cited as the reason the Inspector and Mona drifted apart.
However, it is shown that Benny can go through metamorphosis as well, though he does not change his appearance (due to low budget and plot holes). After the Inspector leaves in the BOOTH, Benny vows to get back to him by whatever means. Shortly after this, we are shown a clip of James Haggard showing up and smiling at Benny.
The popular theory for Benny's return to the show is that it will never happen. The plot threat of Haggard and Benny was left open for far too long and after the writer's change was never alluded to again. However, Hardy and Donohoe (the actors who portrayed the characters) have stated their interest in returning to the show together after working together on Inception.