Brendan was trying to invent liquid fuels for aircrafts. All of the evidence in his lab lines up with this, but his wife, Caitlin, writes in her journal that:"Brendan has managed to fool everyone into thinking he’s developing a new fuel for armored vehicles. In fact, he sometimes conducts tours of the ‘false’ laboratory he maintains next to the library in order to support his subterfuge. If the truth of what he’s developing in the ‘real’ lab were ever discovered…I shudder to think of the consequences."
This may have something to do with the rumor that, while working for the Allies, Brendan was secretly giving information to the Axis during World War II. Because of this rumor, Brendan's brother Eamonn changed his last name to Mallory to avoid association with him.
In 1944, one of Brendan's experiments exploded half of Castle Malloy when he tried using a different propellant combination, killing him and his wife. It was thought that their daughter, Fiona, had died as well, but in 2008, Nancy discovered that she was still hanging around the abandoned castle, flying around on one of Brendan's jetpacks.