What happens in Ludwig Series 1 Episode 1?

John Taylor arrives in Cambridge because his sister-in-law Lucy says that his identical twin, Detective Chief Inspector James Taylor, has disappeared. Lucy needs somebody with James's face to enter the police station and recover a notebook, and the intensely private puzzle setter reluctantly agrees. The deception is supposed to last minutes. Instead, DI Russell Carter collects him for a murder inquiry at a modern office block, where a solicitor has been found stabbed beside his desk after the working day appeared to end normally. John must imitate a brother whose colleagues, habits and job he barely knows while examining a room full of strangers.

The office supplies a crowded field of suspects, including Sarah Gilmarsh. Staff accounts divide the afternoon into innocuous events: a fire door is opened, paperwork is signed, a telephone call delays the victim and people use the stairs as the building empties. Each action looks too ordinary to matter. John sketches the workers' positions and treats their movements as a logic grid. He is less interested in who sounds nervous than in who could occupy every necessary place without contradicting a witness or the physical route through the building.

Three interventions create the killing window. Opening the fire door causes a distraction, giving somebody time to forge the victim's signature. A later call prevents him leaving when expected. Finally, the killer reaches the stairwell at the point when the remaining workers are departing, allowing the fatal attack to be folded into routine movement. John tests the candidates against all three moments. Only Sarah can be present for the complete sequence, and he identifies her in front of the group. The apparent mystery was not an invisible assailant but a planned chain whose components had been reported separately.

Solving the case makes John's impersonation harder to abandon. Carter and the team read his oddness as James behaving unusually after a difficult period; John discovers that applying crossword discipline to people can produce a real arrest. He also sees how much authority James's name carries, which makes every hesitation dangerous. Lucy, waiting outside the official inquiry, has gained a route into the station but also placed John inside a live investigation that can expose them both.

Back in James's office, John locates the notebook that prompted the masquerade. Its contents are not an ordinary diary: the pages use codes and puzzle-like marks that appear designed for somebody able to recognise James's intellectual habits. That discovery changes the family question. James may have left a deliberate trail rather than simply walked away, and John is probably the intended reader. Lucy and Henry still have no answer about where he went, but they now possess the first object that might connect his police work to his disappearance. The episode closes its office murder while opening the larger casebook: John has a solved crime, a borrowed identity and a message from his missing twin that he cannot yet read.

The opening investigation also establishes the limits of John's method. He does not recover a hidden forensic sample or catch the killer in a conventional interview; he makes the timetable understandable enough that Sarah believes her route has been seen. Alice watches a supposed senior officer solve the case with diagrams while Simon responds enthusiastically to every strange instruction. Carter wants a usable answer, not an elegant puzzle, and the distinction will matter throughout John's time with the team. For Lucy, every extra minute at the office raises the risk that a colleague will ask something only James could know. The successful accusation therefore solves the solicitor's death while turning a short burglary of James's desk into an identity John must continue performing.

Who killed the solicitor, and how did John solve it?

Sarah Gilmarsh killed the solicitor. John places every suspect against three linked events: the fire-door distraction that enabled a forged signature, the telephone call that delayed the victim, and the stairwell movement at the end of the day. Sarah is the only person who can fit all three positions.

Key clue: one person, Sarah, can be present at each of the three apparently unrelated interventions.

Red herring: treating the fire door, call and stairwell traffic as separate pieces of office routine.