Encyclopedia knew something was funny about Hagar's inspirational speech. He realized that while Odin and Thor were authentic Norse deities, Cuchulain was a figure from Irish legend. A real Viking would not have included that name in his battle cry.
The man in the dark suit confronted "Hagar," removing his helmet and the large red beard which, it turned out, was a fake one. Under the disguise, "Hagar" was Encyclopedia Brown's old nemesis, Bugs Meany! An elite team of military operatives arrived to deal with Bugs and a messenger was sent to locate the real Hagar the Horrible. By the time Encyclopedia and the rest of the League had reached their safe house, the authentic Hagar was waiting for them.