In his continuing investigation about the Hornets the Doctor goes to 1832 where he sees the titular circus.
Among the guest stars is Michael Maloney who plays very well as Dr Adam Farrow whose sister has come under the influence of the circus.
The ringmaster Antonio is played by Stephen Thorne. Thorne had appeared as various characters in the TV series most notably as Omega in The Three Doctors. Thorne had appeared with Tom Baker before in one TV episode that of the concluding episode of The Hand of Fear in which Thorne played Kastrian Eldrad.
Thorne plays Antonio very well and his style of characterisation is no different to his previous Doctor Who characters.
Intriguing part of The Circus of Doom was when the Doctor comes to the realisation that he may have caused the whole Hornets situation in the first place as the mystery about them continues.