The Doctor may be used to travelling back and forth through time.
Two of the Doctor's former companion investigate a strange head-teacher
But in this week's episode of Doctor Who, the past catches up with him when two of his former side-kicks make an appearance.
Sarah Jane Smith, played by Elisabeth Sladen, and his one time companion, digital dog K-9 feature in this week's story.
Elisabeth Sladen transported herself into the Breakfast studio this morning to tell us about Saturday's episode of Doctor Who - watch again from the link to the right
Sarah Jane's return to, Doctor Who, also sees the time lord reunited with his robotic dog K-9 as well as his former assistant.
After a tip-off from Mickey (Noel Clarke) about suspicious goings on in a local school, The Doctor (David Tennant) and Rose (Billie Piper) go undercover to investigate.
However, also doing her own investigation is Sarah Jane together with her trusty K9.
While Sarah Jane and The Doctor get reacquainted, Rose is less than thrilled with the sudden rivalry.
But together with Mickey the four set out to discover the truth about the mysterious headmaster, Finch (Anthony Head) and his strange curriculum he is infiltrating throughout his school