In "Pathfinders to Mars", sequel to Pathfinders in Space, Conway Henderson (Flood) consents to pilot a new interstellar rocket. Young Geoffrey Wedgewood will be one of the crew and Henderson's niece, Margaret (Hester Cameron) - due for a holiday with her uncle - persuades him to take her, too. Meanwhile the place of Professor Dyson is taken by an unidentified man believed by Wedgewood and the rocket crew to be Dyson. The imposter, Harcourt Brown (George Coulouris), sabotages the rocket's radio receiver so that Henderson and the rest don't discover his real identity. Working for the mysterious "Sector Ten" Brown manages to take control of the rocket and hold Margaret as hostage.