Liverpool 1 focused on Detective Constable Isobel De Pauli (Samantha Janus), a successful Metropolitan Police detective. Her partner's career move to Liverpool in turn sees De Pauli transferred there. Upon arrival she is partnered with introverted DC Mark Callaghan (Mark Womack), a no-nonsense officer brought up on the streets of the city he now polices; the opening sequence of the first episode culminates in Callaghan throwing a suspect from an apartment window.
The earlier episodes centre around the dual themes of the personality clash between De Pauli and Callaghan, coupled with the ongoing saga of the squad's pursuit of local gangster John Sullivan (played by Paul Usher). Callaghan's relationship with Sullivan - who is his cousin - and the interconnectedness of the city, are themes throughout the series.