The episode:  The series was launched with a bang in 1994, with 'The Zero Imperative', which features four past doctors
(although Peter Davison only appears briefly at the end), plus three former companions. Dr Dove (Sylvester McCoy) is the
director of a psychiatric hospital that was scheduled to be closed, until a wealthy industrialist Peter Russell (Colin Baker)
steps in and provides funding to keep it open for another year, although his motives are unclear. The previous director, Dr
Jeremiah O'Kane (Jon Pertwee) was Russell's guardian when he was orphaned at a young age. For some reason Russell
has been compelled to return to the hospital, where he had been raised by O'Kane.  Meanwhile, Liz Shaw investigates a
series of mysterious deaths that have occured near the hospital and believes that there is a connection. It all seems to be
linked to Patient Zero (so named because he is in the room numbered 0 in the hospital's secure wing). Patient Zero is a
conduit for an ancient evil that wants to "come through" to our universe at perihelion, the point in the Earth's orbit when it
come closest to the sun. Patient Zero is in fact Dr O'Kane's brother, Daniel (and Russell's father), who murdered most of
his family over four decades ago. Russell discovers the reason why he has been drawn back to the hospital - he is also a
conduit for the evil force, which needs both himself and Daniel in order to come through to our universe. The evil is thwarted
and Patient Zero dies rather than let a possessed Russell kill Liz. In the final scene the alien seems to have made contact
with the patient in Room One (Peter Davison in an uncredited role). Sophie Aldred appears briefly as Dr Dove's assistant,
and Mark Gatiss features as Dr Bruffin.

The verdict:  'The Zero Imperative' has quite good production values and the script keeps the viewer interested until the end.
Its main attraction film of course is the chance to see three past doctors appearing together one last time.