Doctor Who 50th Anniversary Special Pt 6 Ch 2 'Lost and Found'

The 3rd and 8th Doctor are sent into the Citadel to investigate the Force-Field/Transduction Barrier Room. The 3rd Doctor chooses Jenny as his companion while the 8th Dr asks Chang and Grace to be his. They encounter members of the Outsiders. They discover someone's been tampering with the transduction barrier which protects all of Gallifrey from Invasion. An Old Friend is found.