DI Jack Mooney has a fresh mystery to solve when zoo owner Xander Shepherd is killed by a tranquilliser dart in the middle of the grounds. With the park shut to the public at the time, suspicion falls on the victim's colleagues - but given they were all in the staff area when the shot was fired, how could any of them have done the deed? Meanwhile, Dwayne's replacement arrives in the shape of new officer Ruby Patterson - who also happens to be the Commissioner's niece - and Jack faces a mammoth task when he is signed up to the island's charity swim race.