Peter Lorre makes his fifth appearance as J. P. Marquand's polite but deadly Japanese sleuth Mr. Moto. This time Moto is called in by Scotland Yard to thwart a vicious gang of international assassins. To infiltrate the gang, Moto poses as a Devil's Island inmate and escapes with one of the killers. The climax takes place at the British Museum, where the mysterious leader of the assassins (the least likely suspect, of course) overplays his hand. The gimmick of having Mr. Moto make his first appearance as an apparent villain works only when the "Moto" films aren't seen as a group. On its own merits, however, Mysterious Mr. Moto is one of the best of the series.