Warning: Spoilers ahead for Doctor Who season 15, episode 4, "Lucky Day."Bringing back Millie Gibson as Ruby Sunday in "Lucky Day" isn't just a reference to the events of Doctor Who's Doctor-lite episodes, but also builds the lore surrounding the Doctor's chaotic life and what he leaves in his wake. In fact, Ncuti Gatwa's limited role as Fifteen in "Lucky Day" is what helps reinforce Ruby's big ission.

Although Ruby was the first new companion of various iterations of the Doctor. The dynamic between the Doctor and their companion is one of the show's biggest hallmarks, and although the blueprint occasionally shifts, it certainly has a more recognizable form. What's ignored more often than not is what a companion's life looks like if and when they return to the everyday existence they had before they met the Doctor. "Lucky Day" gives the best insight into that to date.

Ruby Sunday Acknowledges She Has PTSD After Traveling With The Doctor

Millie Gibson's character voices a harsh truth about former companions in "Lucky Day"

"Lucky Day" reveals that, although she appears to have remained in touch with Doctor Who's new UNIT team, especially Kate Lethbridge-Stewart (Jemma Redgrave), Ruby Sunday's life has become comparatively pedestrian when compared to her adventures with Fifteen. However, she's also still not come down from the constant surge of adrenaline that came from her battles against Gods from Doctor Who's Pantheon and other villains. So, in "Lucky Day," the show has her it that she is suffering from Post-Traumatic Stress Disorder.

Conrad uses Ruby's condition against her, manipulating her into believing his ruse in an attempt to prove that UNIT is nothing more than part of a larger conspiracy regarding whether aliens are real.

Mayo Clinic describes PTSD as a mental health condition caused by an "extremely stressful or terrifying event." Ruby's time in the TARDIS was filled with these, so it would make sense that, now the dust has settled, the symptoms are starting to arise. "Lucky Day" portrays Ruby as incredibly paranoid about the Shreek, even though Kate assures her that the creature has been taken care of. Conrad uses Ruby's condition against her, manipulating her into believing his ruse in an attempt to prove that UNIT is nothing more than part of a larger conspiracy regarding whether aliens are real.

Other Companions Have Struggled After Leaving The Doctor's TARDIS

Ruby isn't an isolated case

Ruby is far from being alone in her difficulties when trying to resume her life after ending her tenure as the Doctor's companion. Although David Tennant's Tenth Doctor, but after she left at the end of Doctor Who season 3, she returned as a guest star in season 4 as a part of UNIT.

Clearly, a career without the risk and excitement of life in the TARDIS didn't appeal to Martha anymore after witnessing the wonders/horrors throughout space and time. Similarly, Bonnie Langford's return to the role of Mel Bush in Doctor Who's Disney era provides another example of a former companion being forever changed by their time with the show's main character. Kate even references the phenomenon in general during the alternate timeline in Doctor Who season 14's "73 Yards," stating that UNIT often assists former companions in their transition to a "normal life."

Belinda Being A Reluctant Companion Makes More Sense After Ruby

Doctor Who season 15 made the right choice by not introducing another Ruby Sunday

Those who actually want to travel in the TARDIS with the Doctor often tend to have rough endings or a hard time moving on from that time in their lives. Ruby fits that description, as do many of her predecessors. If another companion of that archetype were to feature after Ruby's departure, it would have felt a little too repetitive, especially after seeing how Ruby is doing in "Lucky Day". So, Doctor Who season 15 replaced Ruby with a different kind of companion by introducing Belinda Chandra (Varada Sethu).

Belinda was abducted in "The Robot Revolution", and although she's grateful to the Doctor for rescuing her, she just wants to go home.

Belinda was abducted in "The Robot Revolution", and although she's grateful to the Doctor for rescuing her, she just wants to go home. Any joy she takes from traveling in the TARDIS is the result of her making the best of a bad situation, as she and the Doctor are being forced to take the "long way round" to her native time. Her resistance to being a companion is a welcome change to the Doctor Who formula, and makes it especially interesting when it comes to how Belinda will handle things when/if she eventually parts ways with the Doctor.

Source: Mayo Clinic