Drugs are big business in prisons. On the inside they can can fetch four times their street value, and serving time can become a business opportunity for the king pin convicts who rule their prison wings. Although prisoners don't have access to funds themselves, family members can pay off the debts their drug-addicted loved ones are racking up on the inside. But to prosecute the offenders, police need evidence of who's involved on the inside - and who's flying the packages in.This film follows the investigation and prosecution of an astonishingly prolific criminal gang who are using drones to deliver drugs direct to cell windows at prisons across the UK.