Who Is DC Caitlin Sullivan in Ludwig?
DC Caitlin Sullivan is played by Rumi Sutton in Ludwig. She joins the Cambridge police team in Series 2.
Updated 8 October 2026
Who is DC Caitlin Sullivan?
Detective Constable Caitlin Sullivan joins Ludwig’s Cambridge police team in Series 2. Rumi Sutton plays her, with credits in all six numbered episodes. She arrives at a different stage of John’s working life from the original constables. He is now a recognised crime-scene consultant, and the officers around him have to decide how to work with his methods.
Caitlin warms to John more quickly than Ethan Cole does, according to David Mitchell’s TV Insider interview. Her response gives his deductions a more welcoming reception within the new pair. Ethan is irritated by a civilian directing police work; Caitlin is sooner prepared to listen. Their different reactions help make the second-series station feel changed even while Carter and John remain at its centre.
John’s behaviour gives both constables plenty to respond to. He can be hesitant with people until a puzzle takes over his attention, then become impatient for everyone to follow the answer he sees. Caitlin’s openness allows those ideas to reach the team without every exchange becoming an argument. Her presence helps connect the explanation John gives with the practical work the officers must do.
Read British Comedy Guide’s full cast record and David Mitchell’s TV Insider interview for the cast and character introductions.
Key episodes
- Series 2 Episode 1 — Joins the theatre investigation as a new detective constable. British Comedy Guide.
- Series 2 Episode 2 — Works with the team as John tests the wedding witnesses’ accounts. British Comedy Guide.
- Series 2 Episode 3 — Takes part in the Cambridge case alongside John and Carter. British Comedy Guide.
- Series 2 Episode 4 — Works the snooker investigation as the team follows John’s reasoning. British Comedy Guide.
- Series 2 Episode 5 — Continues with the detectives during the sushi-bar case. British Comedy Guide.
- Series 2 Episode 6 — Works alongside the team when the latest case reaches into the James mystery. British Comedy Guide.
Full story
Show the full story — spoilers
Caitlin and Ethan become part of the wider alliance against Joanne Kemper at the end of Series 2. Radio Times’ ending explainer confirms that they suspect John is secretly MI5. The belief gives them a way to explain a colleague whose access and authority seem unusual for a civilian consultant.
John is still the puzzle setter the audience knows. The intelligence-service idea is the constables’ explanation for him, and the finale keeps that comic misunderstanding in place while bringing the group closer together. Caitlin can be receptive to John’s methods and still have the wrong impression of how he came to be working at the station.
The group gathers at the newspaper: John, Lucy, Henry, Russell Carter, Gareth Fisher, Ethan and Caitlin. Kemper has walked free after John proves that Ziegler staged his own death to frame her. The team still needs evidence concerning the earlier victims, and James’s whereabouts remain unresolved.
Caitlin’s involvement turns her willingness to work with John into a commitment to the longer investigation. The police case and the Taylors’ private search now have more people contributing to them. The newspaper becomes a place where the different strands can be discussed together, with Gareth pointing out a possible gap in the victim list.
The ending leaves Caitlin on the side of a group that has gained allies without securing a final answer. That gives her place in the ensemble a clear direction: she is part of the effort to keep investigating after a promising accusation falls apart. The larger mystery continues, and the characters have chosen to face the next part of it together.
Follow the finale in TV Guide’s Series 2 ending explainer and Radio Times’ account of the ending.
Relationships
Ethan is Caitlin’s immediate partner and her clearest contrast. Their shared rank places them in the same working situation, but they respond differently to the consultant. Caitlin’s quicker acceptance of John’s ideas makes their exchanges more varied than a pair of officers who always agree. She helps the team move from listening to a deduction towards putting it to use.
Carter provides the senior police connection, while John supplies the unusual expertise at the centre of each case. Caitlin’s role links those two ways of working. The investigation needs an explanation of the pattern and the patient comparison of witnesses, movements and evidence. Her scenes belong to that team effort, giving John people to work with as well as people to address.
Continue with John Taylor, Lucy Betts-Taylor, Russell Carter and the James mystery.
About Rumi Sutton
Rumi Sutton plays Caitlin as one of the second series’ new regulars. The role brings a more receptive response to John into the constables’ partnership, helping his explanations become conversations within the team. Sutton’s character has a useful place in an ensemble built around sharply different temperaments: Caitlin can engage with the consultant’s ideas while Ethan voices the frustration beside her.
That difference gives her more to do than simply agree with a colleague. A team willing to hear John out is essential to a programme where his answer often begins as an unexpected way of looking at the room. Caitlin’s part in that process helps turn a solitary puzzle solver’s insight into work that the detectives can share.
British Comedy Guide credits Rumi Sutton as DC Caitlin Sullivan. Explore the performer directory.
Frequently asked questions
Who plays DC Caitlin Sullivan in Ludwig?
Rumi Sutton plays DC Caitlin Sullivan.
Which series does this guide cover?
The credited appearances listed above cover the character’s first two series of Ludwig; ending details are inside the full-story reveal.