Don Kaplan is a corrupt detective who was involved in the Sonia Stevens case.
He set Liz Birdsworth up to tell the court that Sonia had confessed to the murder of her friend Helen, even though she didn't. Liz's testimony fell through after Helen's phone with a selfie of her and Sonia were found on the morning of trial, resulting in Sonia being released and Liz charged with perjury.
His collusion with Sonia was revealed in the following episode, with Don to be paid handsomely for finding the phone and as a bonus, to be shown where Sonia buried the body of her husband in order to ensure she cant implicate him in the future. On the way to the location, it appeared Don was suspiciously getting very tired, and when Sonia arrives and intends to kill him in order to keep her previous crimes a secret, Don knocks her out cold, knowing that she spiked his drink. Don gives all the evidence required to the police which results in Sonia's rearrest, before taking indefinite stress leave with all her money.